Will transferring retirement extension funds from Bangkok Bank to Kasikorn impact my visa extension application?

Make sure you have bank statements showing all transfers even right back to original transfer from overseas. I had to do this last month.

So can only use bangkok bank to do visa

No problem at all. If you transfer the money in two steps (400k each) you are safe with the requirements, let some days between the transactions, just in cases. Next year you just have to bring two bank statements for your extension to show the continuity.

Only thing you need to be careful is the requirement that your bank account can never drop below 400,000.

So the safest way to make this move is to keep 400,000 in BOTH accounts for a few days.

That means if you have 800k in Bangkok Bank, transfer 400,000 to your Kasikorn bank account and give it a few days to settle there. Then you can transfer the other 400k over.

If you just keep 400k in your Bangkok Bank account during the middle months, deposit a new 400,000 into your Kasikorn account and leave it a few days before withdrawing from Bangkok Bank.

Make sure you do this soon, before you hit the 2 months before you will apply where your accounts is required to be at 800,000.

Note for your extension that you'll need to provide the bank letters and 12 month bank statements for BOTH bank accounts. But next year you will be able to just use your Kasikorn account.