I am a US citizen and am planning on entering Thailand on a non immigrant 90 day visa category O. I am over 50, meet the requirements, income, health, etc for the retirement visa. My question, can I go the immigration office in Pattaya as soon I arrive in Thailand to change the non immigrant visa over to a retirement visa? I will be requesting multiple entries and may want to leave the country for a short side trip 3 weeks after I arrive, so would like to take care of getting the retirement visa as soon as possible.

You can try and I suspect it will be accepted if all paperwork is in order but the normal procedure is during the last 30 days of entry. The is no such thing as "multi entry". You do that by paying 3,800 baht extra and another form to obtain a multi re-entry permit. If you obtain the extension of stay the re-entry permit should not be a problem obtaining.

But I would ask immigration on arrival to stamp you in for 30 day stay rather than use your visa and use the visa entry after your short trip. If they will not do that you have the fall back plan above. Don't enter the visa information on your arrival slip and it is likely they will not have a problem doing this.

See also Non-Immigrant Visa "O-A" (Long stay) - visa for retired persons.