Are you still in the first 3 months of your extension? If not, you can transfer 400k to your new bank then, the following day after receipt of the first 400k transfer the second 400k. Make sure then don't impose the 50k transfer limit because it is they are closing your account so it should be your choice how you do it.
Have you been to your Immigration Office to ask for a list of their requirements and, in particular, if they will accept the combination method for proof of funds?
Yes, just get a copy of your TM30 from reception as you'll need it to get your residence certificate from Immigration. You'll also need a Thai sim registered to your passport.