When you entered Thailand after the DTV rejection, was this an actual Thai Tourist Visa TR60 that you applied and paid for at a Thai Embassy/Consulate??
or, was it actually only a Visa Exempt Entry Stamp made by a Thai Immigration Officer at arrival??
After the e-Visa application has been approved, a confirmation email will be sent to the applicant's registered email. The applicant is kindly requested to print a copy of the confirmation email to present to the airline at the port of departure and to Thai immigration officials in Thailand.
yes, there are reissue penalties, refund charges, etc. on any airline ticket...if one bought a "cheap" ticket, then those type of tickets can't change dates at all....
but, it sounds like the OP wants to stay in Thailand (without having to do a Border Bounce) longer than 90 days, and, also seems that they may not have a Thai bank account yet, so, the easiest solution would be to obtain a Thai E–Visa, then try to re-book the airline ticket (with those change fees/penalties) or get a new airline ticket
if travel plans are flexible and there are no emergency/urgent circumstances, then why not change flight departure to obtain a Thai E–Visa beforehand, then, with a Thai E–Visa in hand, make entry into Thailand