How old is old ?
You can swap out the valves for TRV's fairly easily. The rads on your walls have been bought to suit the size of the room and the amount of external walls, and efficiency doesnt really come into it if they are not clogged. You would probably just benefit from running a system flush for a week or so (or as long as they recommend) draining each rad down, taking it outside and running a hosepipe through it for 10 minutes to get rid of any sludge.
If they are looking dated, then get rid if you are chasing a new 'look', but I installed my combi CH system myself nearly 20 years ago (when you could) and they are still selling the very same type of rad in Wickes now as they were then.
Another thing to note is that TRVs jam up periodically, and a modern condensing boiler is by far the weakest link in the chain. I 'invested' in a Uber efficient SEDBUK A class Viessmann boiler (with the Uber price to match),about 3 years ago, and I've had nothing but problems with it since.