"First, return them to their original form, humans."
Secondly: Give them the genetals of one of the following animals: a horse, a donkey, a dog, a feline, a cetacean, giraf, rhino,bat,... And let them change every so many days or after certain actions, but keep the change pleasant.
Thirdly, whenever they engage in sexual intercourse or activity let them change into full animals or anthros again in various degrees and let their partner adjust to the size, gender and species too.
For the rest add in some quirks of your liking, Beldazzar." Simon said in an amused tone.
"Let them cope with that. Oh man, this will be fun to see."