Hi Jen and Mr. Biddles,
People do not often change from Lantus to ProZinc. ProZinc has a smile curve while Lantus tends to have a flatter, longer cycle. Many cats seem to benefit from the flatter cycle especially if they are big bouncers. ProZinc is popular because it is more flexible - you can shoot at different times, not just every 12 hours, and it is easier to tailor a dose to fit your cat as you don't have to wait so long to change the dose.
Mr. Biddles has a few issues, like pancreatitis and IBD, which can really make this sugar dance more difficult to manage.
@Carol & Murphy has dealt with similar issues and might be able to help.
I also wonder, as Rachel did, if you are getting dose advice on the Lantus forum. Lantus tends to work best when the dose is held for several cycles, not changed often. His doses seem to change quite often. He seems to have been on Lantus awhile and with his multiple issues (and probably meds for those) he may be harder to regulate. I wonder if you are following their protocol for optimum results? Have you posted there and asked for help, emphasizing his other issues? Their forum works best if you post daily before the shot, getting dosing advice and raise the dose when the protocol indicates.
@Bobbie And Bubba changed to Lantus a couple months ago and might be able to give you some guidance.
If he continues to need more insulin, you might also look at Levemir, which is said to sting less at higher doses. He might have a higher dose condition, but it is early to suspect that.
If you decide to change, as Rachel said, we will be happy to help.