I've seen this discussed on the forums before.
Essentially, you were granted an extension of stay based on the 800k rule. So when you turn up for your next one, even though you want to switch to income method, they will expect you to have complied with the terms you were granted on your existing one. This means keeping the 800k for 3 months after.
@DrJack54FYI, I used to use 2 separate accounts although they were both at the same bank. I was asked to consolidate funds to one account about 5-6 years ago, but extension was not refused. I think you'll manage to get by with the 2 separate accounts under the circumstances. Just have a backup plan, leaving time to use an agent is maybe the option of least resistance, good luck