Can I extend my Visa on Arrival in Thailand from 15 to 30 days?

Currently visa on arrival get 30 days. Due to change on April 1st back to 15 days (but may not - we just don’t know as yet).

Passport holders from Fiji qualify for Visa on Arrival. I believe there is an option to fill out the application online before you arrive in Thailand, but this is not called evisa, though I'm unsure what the name is.

Normally this is for 15 days as you stated, but if you arrive before April 1st, this has been temporarily extended to 30 days. So you'll already receive the 30 days you are looking for if your trip is before April.

Visa on Arrival does not qualify for an extension unfortunately.

What country is your passport from? There's no such thing as a 15 day evisa so knowing where you are coming from might help us figure out what the actual situation is.