can u unmemorise spells - or change what type of familiar u get?

Do you mean uncopy spells from your spell book? If so, then no (although there may be a cheat I'm unaware of). Also, you cannot change your familiar. It is dependent on your alignment. The list of familiars and their corresponding alignments is at the bottom of the spell description. (are you neutral and bummed about the rabbit. I was a neutral mage my first runthrough - BG1 character - and I just blew off the familiar. My wife (CN F/M) had a fairy dragon, which is the best you can get, but toward the end of the game it got to be a pain in the butt more than anything else.)

If you need to forget spells to learn new ones, because you have reached your limit, drink an intelligence boosting potion just before scribing the spell. This way you can learn any number of spells and access level 9 spells if you are normally INT 17 or less. Download Shadowkeeper, from most BG2 sites, as this can allow you to hack your character and change anything including familiars and spells, but be careful as it can spoil the fun if you abuse it.