I also agree the diarrhea is ECID. Cats metabolism is very different than humans so I just don't know if Lantus could cause Mocha to have diarrhea but I never say never.
Levemir is a very nice insulin and it was a much better choice for Gracie but it didn't stop her bouncing entirely. Her cycles are much flatter.
As Wendy posted on your Health condo, lev is usually started at 70% of the Lantus dose and it is very important to be extra vigilant on testing when you first switch as you will give Lantus one cycle and levemir the very next do one depot will be depleting as the other is starting to fill. You might not see a lot of action.....you might see high, flat numbers. It's really ECID but it's smart to do a little extra testing.
I've been very happy with the switch but IMHO, I think I'd be working with my vet to try and rule out other causes of diarrhea which could include intestinal inflammation or IBD, parasites, developing food allergy, etc.