Is it normal to be charged ฿2,500 when changing the reason for a Non-Immigrant O visa renewal from marriage to retirement?

When you do an extension for the second year in a row, you can, at no extra cost, switch from, for example, retired to married or vice versa

There is now a 500 baht 'labour' charge at immigration offices for most visas.... Its up on a sign on the wall

pretty far from a "quick question" 😕

You're not changing a Non-O to anything, 😕

If you are on a yearly extension based on marriage to a thai, and you want to switch to a yearly extension based on being over 50 (retirement) it's the same cost as all extensions inside thailand 1900baht ..

All you're doing is changing the reason for the extension when it comes up for a new one from one reason to another.

Keep in mind when you do a 'reason change' on an extension they don't add the new year to your current expiration date, they start it the day you apply, so if you went early (before your current extension expires) you usually lose those days