Thanks for the replies. I made a BDB (Bluray Drive Bypass) PUP of fw 3.50 and managed to update it. 3.50fw is supported by the latest remmary files on ps3devwiki so hopefully the remarry process will go well.
Just waiting on a replacement bluray drive, should have it in a few days.
If anyone is wondering about the 'right way' to do it, I think JustThatDude is correct. Replace the bluray drive first then do the downgrade if needed then do the remarry.