There are 8 spells of 4th level for Illusionists in AD&D, the same number as they had in OD&D. Two of the spells on the AD&D list were 3rd level in OD&D: phantasmal killer and dispel exhaustion. They replace shadow magic (which gets bumped to 5th level) and 1st-level magic-user spells, which is bumped up all the way to 7th level.
Confusion: Like the 4th-level magic-user spell and the 7th-level druid spell, it causes all those affected within the area to act at random, as determined by rolling on a table. It has a range of 8" (vs. 12" for magic-users), a duration of 1 round/level (magic-users get the same, with 1 round extra), and an area of 4"x4" (vs. 6"x6" for an m-u). Illusionists have the same parameters for this spell as druids (except for a casting time of 4 segments, vs. 9 for druids).
The OD&D spell gave no saving throw for creatures under 4 Hit Dice, where as this version has no such power. It also had a delayed effect on any creature over 2 HD; again, that's not present in the AD&D spell.
Dispel Exhaustion: This spell restores 50% of a target's hit points, affecting up to 4 creatures touched by the caster in a single round. It also allows a target to move at double speed for 1 round per turn. This effect is illusory, and when the spell ends the targets all drop back to the hp total they had before (modified, one assumes, by any damage taken in the interim). The spell has a lengthy duration, though, so it's actually worth using in lieu of genuine healing.
This spell was 3rd-level in OD&D. It didn't mention restoring hit points specifically. Instead it "allowed action without rest", but after the spell the recipient had to rest for twice the amount that they missed. It allowed those recently raised from the dead to act normally, as well as those badly wounded, but they took 1d6 damage while doing so. It had a flat duration of 4 hours, whereas in AD&D the spell lasts for 3 turns/level.
Emotion: Causes all creatures within a 4" x 4" area to react with one of four emotional effects. Fear causes them to panic and flee, like the fear spell, but the targets suffer a -2 penalty to their saves. Hate grants the recipients +2 to morale, saving throws, attack rolls and damage. Hopelessness makes its targets utterly dejected. They will turn back from any task, submit to any demands, and have a 25% chance of doing nothing at all in any round. Rage drives the recipients berserk, giving them +1 to hit, +3 to damage, and +5 to hit points, but causing them to fight with no regard for their own safety. Fear and rage counter each other, and the same goes for hate/hopelessness.
The OD&D spell was the same in principle, but its practical effects were completely different. Fear was much the same, mimicking the OD&D version of that spell. Battle lust was the equivalent of rage, but its mechanical effects weren't quite so extensive. It simply allowed the recipient to fight as a Berserker, granting them +2 to attack and and making them immune to morale checks. (I suppose it could have been referring to the Berserker class from Dragon magazine, which would grant more extensive benefits.) Fear could counter battle-lust, but not vice-versa. Deprivation was the equivalent of hopelessness. Bravado granted immunity to fear, and could counter deprivation. Hate didn't grant any bonuses. Instead it made the victim attack a random target, as rolled on a chart. There was even a result for "hate self", meaning that they would commit suicide. It's not hard to see why Gary tweaked that. Also, the OD&D spell never specifies how many targets or what area it can effect. It's completely open to interpretation.
Improved Invisibility: This spell works like invisibility, except the target can make attacks and cast spells without reappearing. They can be detected by a tell-tale shimmer, but all attacks against them suffer a penalty, and they gain a bonus on all saving throws.
In OD&D, the spell was said to be the same as Invisibility 10' Radius, which would have meant that it could affect numerous targets instead of the single target of the AD&D spell.
Massmorph: Like the 4th-level magic-user spell, it gives a number of creatures the appearance of a copse of trees. Unlike the m-u spell it requires no material components, and only takes 4 segments to cast (magic-users take a full turn). It affects a 1" x 1" area. Weirdly, the magic-user spell affects an area of 11' x 1". It's a very odd measurement for a D&D spell, and I wonder if there's perhaps a typo.
The major difference from the OD&D version of the spell is that in OD&D it affected a flat 100 man-sized creatures. The AD&D spell affects 10 per level.
Minor Creation: The caster can create a non-living, organic object of 1 cubic foot per level in volume. This generally means things made out of wood, ropes, and "soft goods", which apparently means cloth and fabric. The items last for an hour per level. Material component is a small piece of the same kind of object being created.
The OD&D spell creates the same kinds of materials, but is limited by weight rather than volume. It also has a duration measures in days rather than hours, with a nebulous bonus or penalty to be applied by the DM based on the hardness of the item created.
Phantasmal Killer: This spell creates an image in the target's mind of the most horrible creature that their subconscious can dredge up. This image attacks as a 4 Hit Dice creature, and if it strikes a blow the target will die from fright. Saving throws against this spell are rolled in a nonstandard manner, being a 3d6 roll under the target's Intelligence (with various situational modifiers applied). My favourite part is the note about helms of telepathy: not only does wearing one grant a -3 bonus to the roll, but it also allows the target to turn the phantasmal killer back on the caster.
The OD&D version of this spell was 3rd level, and I can see why they bumped it up. The spell works almost exactly the same, except that the various modifiers to the saving throw are different. Having been attacked by this spell previously grants a flat -5 bonus, whereas in AD&D it's -1 per previous attack. The OD&D spell had a range of 6", as opposed to 1/2" per level in AD&D.
Shadow Monsters: Creates a number of semi-real monsters, whose total combined Hit Dice cannot exceed the level of the caster. They only have 20% of their usual hit points. Against any target that fails their saving throw, the monsters defend and deal damage as normal. Against those that make their save, they will have AC 10 and deal 20% of normal damage.
The OD&D version of the spell also summoned semi-real monsters, with much the same parameters. All those monsters had an AC of 9 (the worst possible in OD&D), regardless of whether the targets believed in them or not. The spell was much more specific about monster special abilities, such as breath weapons and petrification: they only worked on those that believed they were real. AD&D is a bit vaguer, saying only that they "perform as normal with respect to armor class and attack forms". Shadow monsters in OD&D took double damage from silver weapons, a weakness not present in AD&D.