It is true that your Non-O must be canceled before you get the LTR stamped.
I did it in Bangkok, but this was also the office where my Eos for the Non-O was done. I was informed that you have to cancel the Non-O were it was approved or extended. If this is 💯 true, you never know...
Ask BOI or just give it a try in Bkk.
You have to provide a bank statement which shows that you fulfilled the financial requirements for Non-O all the time you had it since last Eos. No idea what happens if you cannot.
Or, if you do not have a multiple entry permit, just leave Thailand. This will automatically invalidate aka cancel your Non-O.