you can change to a single non o inside Thailand, you need at least 15 days balance and have all documents ready. Including financial, however not every Immigration can do. Bangkok is OK.
no agencies that do Savannakhet and no agency in HCMC. However process is straight forward. Both places do not require "show money" in order to get the multiple Non O ( in case you referred to that visa).
the problem is not an easy airport in another country the problem is that you enter Thailand under visa exempt, the fourth in a row, if I would be IO I would grill you, you might consider applying for a tourist visa at an easy consulate and come in on that 60day Tourist Visa.
I think OP has all the option, if there is an issue with checking in than you better get a 60day tourist visa, if a Thai Consulate is nearby. Or just enter Thailand on 30 day visa exempt stamp and get a 30 day extension or do a side trip.
I deleted one comment referring to a person of the past. Considering that various person and agencies follow this page I think it is not appropriate to call people with 'nicknames".