I was hoping that someone who has actual experience with a Prisma would answer your question, but that does not seem to be happening.  If you watch the video illustrating the Prisma Fit upgrade from Siemens (follow the link below), you will see that everything except the magnet is replaced.  Since the new Prisma magnet is a "Zero boil-off" magnet, I think the primary difference you will see between the two is the rate of liquid Helium boil-off.  Since the gradients, RF system, and software are identical I would not expect any noticeable performance difference.

https://usa.healthcare.siemens.com/magnetic-resonance-imaging/options-and-upgrades/upgrades/magnetom-trio-upgrade/features