M experience is also over a year old, but I have done this a couple of times in the past. Here’s my experience: If you already have a Non-Profit Trial set up, for the 25 licenses maximum, they were never, ever, under any circumstances able to grow this trial number until the non-profit account was validated, nor could they make it a commercial account. They could (with some effort on the phone with support) extend the trial a month or two longer.
For an account that starts out commercial that you want to convert to non-profit pricing, my experience with this was you had to open a support case with Microsoft and they would start the investigation process, and eventually once validated, the non-profit status would be applied, but you could continue as a commercial account until such time as they got the validation done.
So it depended if you started the “non-profit trial” or a commercial trial/account that you later wanted to convert.
Like @codatory, I don’t have recent experience to say how they are doing it now, but I do know that the non-profit trial that leads to validation was extremely strict and they were unable to do anything useful with the account until it was validated, except extend the trial period once or twice. The 25 trial license limit was an extremely hard limit.