Replacing radiator query

I am looking to replace a rad in a room with a bigger rad as it is too small. I asked a plumber and he said I would have to flush the system after - hence higher price. I though it was just a case of closing off valves, pouring out contents, and sorting the fixings to the wall. Once the rad is replaced and valves etd open is there anything else that needs to be done? I know there's the valve that you open to increase the pressure in the system and I was assuming I might have to do this if, because of the increased rad size, the pressue at the boiler drops. Any tips?

what system have you got? is it pressurised and whats the boiler, how old is it, flushing the ssytem is actually a good idea and to make it even more efficient you should add a rust inhibitor, the name of which escapes me but i use it.

If it is just a radiator replacement and there are no other issues with your heating system, i.e. circulation problems, sludge, etc. then there is no need to carry out a powerflush when replacing a radiator, especially if the only reason for replacement is a sizing issue.
If you are changing the radiator size from a single panel rad to an identical width one but double, then replacement is simple. If pipework needs to be extended to accommodate a wider radiator, then the system needs to be either drained for pipes frozen to install.
Make sure a decent inhibitor is put into the system either way such as Fernox F1.