if you do not put it back up to 800k 2 months before application AND if for some reason they deny your application of income method, you will not qualify for the extension.
It would be prudent to top it back up if you are able, for a plan B just in case.
You never know.
And remember the 800k must be in the account for 3 months after approval NOT day of application.
Assuming you are on the 1 yr extension and not the original 90 day visa.
In the best case scenario she (and you) will have all the documentation ready to apply AT THE SAME TIME/DAY as she cancels and they will issue a new 90 day visa.
IF you are not prepared you will need to leave to a neighboring country and apply from there.
Best to speak to your local office to make sure they allow this type of change.