my radiators are quite old,been in use with an old convetional boiler but iv'e had a gas fitter round and he says it will be quite all right to fit a new combi boiler, and use them. be grateful for other opinions.
Then ask them why they insist on doing it and see what answer you get.
There are different qualities of radiator (thickness of the metal they are made of) - old ones are much thicker. To be absolutely sure you won't have a radiator leak problem that they will feel duty-bound to come back and fix, some plumbers will insist on changing the lot. It also creates more business for them, for course.
Ask the one who doesn't think it is necessary to explain to you why.
if it was a money issue i wouldnt worry about replacing them but newer rads are more effecient & often smaller and while the system is drained down its quicker and easier to replace them.