Is it possible to change the renewal date on a Non-O retirement visa in Thailand?

No, then you need to start with a new visa.

So you’re saying that when you apply for the Spanish visa you have to send off your passport so won’t have it in your possession when you need to do your extension???

Non-O extencion (renew) can be done 100 days before it expires and will continue from the date it was originally

settled. I made it last November and it continues from February from original date one year ahead. So it was that time on November valid around 15 months.

I have had someone tell me I was wrong for saying that you can renew at any time if you have a valid reason but the visa will run for 1 year from date of issue unless it is withing 40 day of expiry of the old one. I was told this in Immigration when I went to renew my visa in Pattaya before trabeling up to Khon Kaen and not knowing if I would be back in time as it was I was 38 days from expiry so it was a normal reneway. Also my brother who is also on a retirement visa extension had to return to UK for medical reasons a few years back and he was about half way through the year and was expecting a long stay and again he was able to renew his so it is possible but probably down to the Immigration officer is he/she thinks it's acceptable.

Just ask the Immigration, they are all different some might do it some not, Mine Did.

Can you be more detailed please @Joan Watson? Are you already on your first 1 year Extension which expires? Or is your 90 days non-O visa expiring?

You can't change the date your annual extension runs to unless either you apply for an extension and don't have 12 months validity on your passport (in which case it would change to match the validity date of your passport) or you exit Thailand without a re-entry permit and start over with a new Non-immigrant visa

When is is due ?