Ian's bases are excellent and very well cast, so just pick up the ones you like best. He's also fast with shipping and such.
As for rebasing them: are they on slottas or integrals? Either way, you should start by popping them off their bases (although you might have to utterly destroy the bases to keep the minis and paint intact) and then removing everything under their feet. Then simply pin them to the new bases.
Working with painted models is a bit risky, especially when drilling or filing. Try to wrap some soft cloth or something around them where you hold them, and work slowly.