I definitely see why people find it difficult to come to terms with the fact that some medications for Crohn's disease come from animal sources and contain animal ingredients. Unfortunately, every medicine currently in use in the US and UK will have been tested on animals at some point, there is just no way to test new drugs safely without monitoring the affects on animals first. This has always been, and will always be a huge debate and its not something everyone is comfortable with. I believe animal testing should only be done when medically necessary (so not for cosmetics etc) and should be done as humanely as possible, but if we start going down the road of not using meds because of animal involvement we will all be in very serious trouble.
Regarding switching from one version to the other I'm not sure, they are both still rodents and it's possible they'd be so alike that the body would react to both in the same way.
In terms of vegans, I do know of a few vegans on infliximab or the biosimilar, most seem to feel that when its a matter of health it is acceptable to use animal products. Most vegans I know directly understand that for many people it just isn't safe to never use any medication in their life. If this is something that does concern you please reach out to your doctor.
