When you remove a level from a character, the skills are not changed to match the new level but the old level entry is removed from the level list on the skills tab. As a result the user cannot correct the skills without editing the PCG file.
For each level removed:
Remove a rank from each skill with max ranks at the old level (now above max ranks)
If the remaining skill point pool (remainder) for the level being removed is not zero:
If there are other levels left of the class being removed, subtract the remainder from the remaining value of the next highest level of the class removed.
Otherwise subtract the remainder from the remaining value of the character's highest remaining level.
Skills now get dealt with as described above.