How can a UK Passport holder manage multiple passports and visa extensions in Thailand?

Several questions for the price of one.

UK Passport holder on a non-imm O extension of stay for retirement valid till 28/11/19

Passport has 2 years till expiry and 5 blank pages.

Second UK passport never used. Valid for 2 more years and want to start using it next trip (as yet unplanned).

At next renewal I will lose 2 pages (new extension and multi re-entry stamps) so down to 3 blanks.

I understand to start using my pristine passport I need to leave on the one I entered with and return on the new one. But how do I book a return flight with the outward journey on the current passport number and the return flight on the new number for boarding pass purposes?

Also, I am considering changing the reason for the extension from retirement to marriage, and I realise the different requirements for this. However when can I change this? Can I do it anytime or only when I process my renewal which this year will be around the end of October 2019?

Finally as my second passport is unused but technically not new, can I ask the Immigration Office to transfer current stamps to my second passport or can this only be done for voided/new passports?

Thanks