How much work is involved to change a conventional boiler to a combi boiler?
will we need work done in every room?


I haven't done this but I'm pretty sure that most rooms do not need to be touched. Only


The boiler location obviously
The airing-cupboard - removal of hot-water tank, replumbing.
The loft (attic) - removal of header tank for hot-water etc.


You are changing from this



to this



Images from wellwarm plumbing - no affiliation

The diagrams above are the best I could find but have a couple of anomalies:


the pump and zone-valves are usually next to the boiler, not next to the hot-water tank.
I would expect to connect to existing hot tap pipes in airing cupboard, not run new supply pipes to a different location.


Details obviously depend on the exact arrangements in your home - but in general, I can see no reason to alter anything in other rooms.