We have a long checklist for each upgrade, with a couple hours of things to go through (building the project scope) before putting hands on keyboard to start the work. 

Knowing the customer system in advance would simplify things for sure.

Look at what your last upgrade took and be prepared to double it at least since you have payroll, multibin and credit card processing.

I agree have the partner do it for the payroll upgrade there is a lot to it.  If you get the tax tables or the earnings and deduction codes wrong it can cause more work than the original upgrade.