Thanks to all who responded, the concensus seems to be that most warbirds keep their military serials…..most of the time…..

Sorry to join this discussion late, but I think that there is no regulation which requires a warbird to wear its original serial. The CAA give dispensation to wear (any) miltary marks, and the MoD approves the specific one that the owner wants to use. For instance the Hurricane (ex RCAF) which crashed last year was wearing its second fake RAF serial, and for a while had worn none at all. Frankly, I wish they would always wear the original serial, even in museums – it would make keeping track a lot easier!