When I was forced to convert my old 1990's ppl
You didn't
convert your 1990s ICAO UK PPL to obtain a LAPL. In the past the CAA have been guilty of stating that it was a conversion but they were wrong to do so. If they took your PPL and didn't return it to you then ask for it back. It was issued to you for life. Hopefully you will still have your old PPL. If so you can solve the medical issue by making a 'Self Declaration'. The PPL privileges you should hold gives you all and more than the LAPL so why not. You will, of course, have to do a renewal test to reactivate your UK ICAO PPL but this will only require one problem to be solved. The Self Declaration is of course free! This saving should cover you renewal test.