@Tong ****
There was a total miscommunication between what you said you have in the original post and what you're really on 😮 and that caused this thread to very nearly come off the rails
Reading the stamp IN your passport would have showed you that you are currently on an extension 😕 not any visa 😉 Your stamp now should say "EXTENSION OF STAY PERMITTED UNTIL" and have a date on it that it's good until
SO after all that it turns out you CAN indeed 'change the reason' for your up coming extension without any issue at all. (y)
As long as you met the requirements for the year extension you're currently on based on retirement and meet the requirements for the new extension you'd apply for based on marriage you should be fine 🙂