Game already warns you or warned you before, that if you move from Y+2 or regular +2 to different lower level skills like Y and Y+1, you will loose the skills for Y+2 or any that is higher from the one you switched. If you switch skills, you should only switch to the regular +2 or whichever is the maximum upgrade for that skill, or to Y+2, or any other that is maximum for that section.

Hope it cleared up? Unless you have different bug where you should describe problem more clearly, since it seems you switched to a lower skill and now you need to get those skillbooks again. I know it is a * feature that you can't switch to any skill and you need skillbooks to get back up, I see no point in it except to punish player in pretty * ways.