the corgi guy has to do a saftey test on the whole system to check for leaks - before and after the fire swap - then he has to comission the fire (correct flame height etc) and check the flue is ok - its not just a case of unhooking one and hooking the other up
it is 'technicaly' legal to do it yourself, but if anything goes wrong, you have to 'prove' you are 'competant' in gas fire installation (or cookers or whatever you have done)
you 'have' to be corgi certified to do it as a trade
similar to the new electrics rule