When I come to renew my retirement visa I'll only have 5 months validity on my passport so will presumably need to get a new one prior.

Should I then get visa and stamps transferred or wait until I apply for the new visa?

If you do not get a new passport your extension of stay (it is not a visa) will only be issued to the day your passport expires.

After getting a new passport you could wait and apply for the extension and stamp transfer at the same time.

But in my opinion it would best do the stamp transfer first if it is going to a considerable period of time before you apply for the extension.