We went from open-vented system to combi with no problem. 22mm main flow and return running the length of the attic with 8mm off to each radiator. We didn’t have those daft single radiator valves – they are bobbins. Did have a magnetic filter installed and a powerflush at the same time.
Can’t see any reason why new boiler and better rads would be a problem, although new system throughout would be better. Depending on size of house and ease of access under floors it shouldn’t be that much more work.
Source: am heating engineer.