Apologies, a bit late coming in on this one....just wanted to add my experience to the thread...
I went to the main office on 8th May, 42 days before my extension expiry date.
I handed over my British Embassy letter from December (which I had previously been told by an IO in the satellite office would not be accepted after 1st March). I accept that things change and senior Police Captain Nunsoongnern (I think) that I saw was happy to take it. He also took my Kasikorn bank letter and 12 months certified bank statements. I had highlighted the 65,000 Baht transfers which I have been making on the 1st of each month since the rules changed. Prior to that I highlighted any combination of transfers that totalled more than 65,000 Baht p.m.
I also had TransferWise PDF printouts for each highlighted transfer but he wasn't interested in those. (On the Thai version of the bank statement TW payments show as 'cheque paid in' or something similar).
So......I am sorted for this year but I still have no idea how I will get on in June 2020 because I will not have the embassy letter and I am pretty sure that Kasikorn bank will still not be able to issue a letter that categorically states ("guarantees") that I receive a minimum of 65,000 Baht p.m. from overseas....... Which is what I think Immigration want to see (I have seen such a letter issued by Bangkok Bank).