I have a multiple O based on marriage which is not due to expire until September. With Visa runs, my passport is likely to run out of pages before then. I am returning to the UK for a holiday in June and my plan is to get a new passport while I am there. On returning, is the visa in my old passport still valid, or do I need to get a new one?
TLDR : Answer Summary
If you have a multiple O visa based on marriage and plan to get a new passport while in the UK, your visa will still be valid, but it is crucial to keep your old passport with you. Upon returning to Thailand, you will need to have the visa transferred to your new passport. It's advisable to check with the authorities in the UK to ensure they do not keep your old passport during the renewal process, as you'll need it for immigration in Thailand. Additionally, consider applying for the new passport before your trip, and ask for a letter confirming the change in passport numbers if you can.