Replace boiler and leave CH system as is?

I think the official answer is that you must upgrade the controls.In practice you may find someone who'll turn a blind eye and just replace the boiler, but they may be dodgy in other respects too. Isn't it possible to put inline TRVs in the pipework to each rad leaving the existing valves in place (but rendered not manually adjustable so they're just used for balancing)?

The inline TRV's seem like a good idea to me but if I'm going to the trouble of doing that, I might as well do the full modification. This being so, is it realistic to mix micro bore and 15mm pipework in the same pipe run, or would it be necessary to replace micro bore with

Yes go ahead do what ever looks best. The only no-no is to start adding radiator plumbed entirely in 15mm straight off the 22mm main flow and returns, even then you can still make it work but you'll have to screw the lock shield valves right down.

Thanks Ed, glad you cleared that up. The no-no was exactly what I was going to do until you pointed it out.