Unless you have cause to think the crusty pipe connections may fail I'd suggest you don't fix what isn't busted.
Do note that installing an unvented cylinder should be done by a competent and certified installer. Incorrectly installed it is a potential bomb.
I think there's a requirement for building regs approval for swapping a vented one now as it comes under conservation of energy but iirc prelagged complies as long as the temperature controls are up to date and immersion stat is compliant.