IMO, most gimmicks are obvious almost right away (and indeed, that's how I intended them to be), with the obvious exceptions of the ones first used in Sneaky 4 and Playtime 10 (and, in a sense, Dodgy 18). The player still has to work out how to deal with the gimmick (or in many cases, actually make use of it rather than just working around it), as well as work out the exact details of how it functions - all they're given in most cases is a brief overview of the concept.
For LPII, the levels that were modified are:
Cheeky 2 - Tidied up the one-way arrows
Cheeky 3 - Tidied up the one-way arrows
Cheeky 8 - Majorly decreased the difficulty (save requirement reduced to 40%, skillset increased to 50 of each)
Sneaky 6 - Tidied up the one-way arrows
Cunning 9 - Replaced fake exit with the fire object
Cunning 18 - The entrance on the right side is now left-facing
Cunning 19 - Majorly increased the difficulty (save requirement increased to 100%, first four skills reduced to 0 each, last four to 2 each)
Genius 2 - Somewhat increased the difficulty (save requirement increased to 100%)
Genius 13 - Somewhat increased the difficulty (save requirement increased to 85%; yes, this is possible on the NeoLemmix version*)
Genius 20 - Slightly moved the exit position to keep it in line with certain terrain features
There's one other level too, but I'll leave that one as a surprise. It's very noticable.
* I actually can't think of any reason why this 85% solution wouldn't work on the traditional Lemmix version too (give or take 1%), though I didn't try it. If it does, then this is a MAJOR oversight by all of us - the current record stands at 79%! EDIT: I checked, and it won't work on the traditional version due to a difference in the skillset (which was nessecary because the traditional version's solution won't work on the NeoLemmix version due to differences in builder mechanics). However, I can confirm 86% is possible on the NeoLemmix version, though I don't plan to change the requirement to this, it's fine as it is.
For the bonus pack - I haven't actually made the modifications yet, just identified where they're needed (and this doesn't include Rush or secret levels as I haven't looked at them yet):
Reverse 6 - One less builder (as it's not needed in the NeoLemmix version)
Flight 3 - Tidy up the one-way arrows