I presume you're talking about an extension of stay obtained in-country rather than a visa gotten from an embassy outside thailand
Yes - you can change the reason of having an extension of stay (in your case from >50 to marriage)
As for the documents- I'm not 100% sure but probably best to say which immigration office you're using or visit them and get the checklist or do a group search for extension of stay - marriage.
Have a look at this post about 'retirement' visa -
If you don't want to go back to UK to get the non-O-A, you may (depending on the immigration office) be able to come in on a visa exempt entry (or tourist visa) change to non-0 (>50yrs) and then onto yearly extension. (Or even miss one step and apply for the non-0 in hkg)
One day, - today / yesterday immigration were closed. Go tomorrow (the first next working day) get an extension for 1900, it'll be back dated but you'll clear any 'dangers' of being in overstay.