It depends largely on when the extension was built, because thermal insulation regs have become more stringent over time.
If you can show that the thermal performance of the extension complied with the regs in force at the time, it could be that it can be classed as a normal extension rather than a conservatory/highly-glazed extension.
If that's the case, you could keep the existing doors and rad.
But you will in the meantime need to find out when it was built, and also to read up on how to do U-value figures.