If you know you are going to stay 40 or 45 days why not simply buy your expected departure date flight and a few days after you arrive go to an io anywhere in Thailand and buy a 30day extension it is easy and should take no more than 1 hour, just go in and ask for an extension, they are fully prepared for it and know what to do, I do this every time paying an agent for something as simple as that is a complete waste of money as is paying a fee to change your flight.
One word of caution, tea money is still very much a thing with io’s so hand over 2,000baht and do not expect or ask for any change, if the io offers change (unlikely) don’t accept it just quietly tip it back over the counter they will remember you next time
just go for it you will be surprised at how easy it is.