If you choose to get a NPPL SSEA, you don't exactly downgrade your PPL to it, you would hold it in addition, so have two licences, therefore there would be no upgrade one the medical issues are sorted.
The NPPL would be issued on the basis of your existing and current PPL, just the administration fee and medical declaration to pay for.
You can also use the PIC hours gained using the NPPL to keep the SEP class rating on your PPL current, if that's getting anywhere near its expiry date.