Just some constructive comments.

Why don't you run a test build. Copy the database onto a seperate SQL server. Host it under a subdomain then you have two copies of the data. One to work on the upgrade while keeping fresh up and running. Then when you've got the majority of the bugs tweaked. Open up the test server to other people for stress testing and finally once all the processes are complete do an upgrade?

I can understand money being an issue but it would at least take the pressure off. You could test the upgrade at your leisure without spending days and days at a time doing long hours when you could feasibly do the upgrade a few hours a day over two weeks. Once you're happy just power through the processes on the main server?