There are any number of potions out there to turn Sims into (or cure Sims of) vampire and plantsim, such as:
http://www.insimenator.org/showthread.php?t=38901
http://www.modthesims.info/download.php?t=201384
(Lots of others which regular searches will turn up.)
Or one can use objects (such as Insimenator) to change them or to change them back.
But gonna give you links to some more unusual items/methods (besides the plethora of potions).
Sim Transformer: http://www.modthesims.info/download.php?t=283201
Plantsimenator ($$): http://simslice.com/thesims2/guests/wintermuteai1-pg5.htm
Or at the booty (simslice-ts2-plantsimminator.zip): http://paysites.mustbedestroyed.org/booty/ts2/simslice/wintermuteail-guest_artist/
Vampire bat edibles ($$): http://simslice.com/thesims2/ts2objects-pg9.htm
Or at the booty: http://paysites.mustbedestroyed.org/booty/ts2/simslice/objects/vampire_bats/
I'm kind of partial to the vampire bat edibles, as the occasional hungry visitor will nibble on them, too.
As for becoming a plantsim without using hacks, use herbicides enough and keep aspiration up and it'll happen.