@Malcolm ****
for retirement extension you need to wait until at least 3 months after your extension, because the requirement during those 3 months is that your account remains at 800,000.
Then after that point, when the requirement is just that you must have 400,000 in your account, you can transfer 400,000 to your new account. Give it a few days to settle, and then transfer the other 400,000 into it. This is so you can show you had at least 400,000 fully settled into an account at all times.
Next year for your extension you will need to get the statements and bank letters for BOTH accounts. But after that you can just do everything with the new account in following years.