I believe that you can make copies of the files that the patch replaces and then install the patch so that you can just copy those files back in case you want to remove the patch. If you didn't do that then yes, you CAN copy your save files into a different folder and then reinstall but if you use the uninstall option it'll leave your save files intact.
As for learning the spells, becoming an assassin would affect your ability to learn spirit spells. You should already learn your vine spells at half of the normal level. When you change roles you can still learn spells from previous roles, but at half the normal level. Example: Joe was a priest with level 5 Vinecraft and level 6 spirit. He changes into a monk which still allows him to learn spirit magic at normal level because spirit is a spellbook normally learned by monks. Vinecraft however isn't, thus it is learned at a level equal to one half of his vinecraft skill so you divide 5(which is his vinecraft skill level) by 2 which equals 2.5 which is always rounded down. I hope that didn't confuse you
.
So changing to an assassin will affect your spirit magic from what it currently is but since Vine is already learned at half of its current level so that'll stay the same. I would recommend learning Rebirth before changing to an assassin unless you already have someone else learning spirit magic since you'll never be able to learn level 7 spells, only up to level six (skills can only go up to level 12 and half of 12 would be 6).
-Jafin