If you are performing an upgrade on a customer using 4.3 or earlier I've found a high incidence of the registration keys not working. It seems that one cause is the new keys were generated under the old customer number.
Call support right away. They have a way to test the keys (why this is not on the website or why they don't run all customers through a test is beyond me).
Sage support may offer a workaround of just using the old customer number with the new keys. Don't fall into this trap since Aatrix relies on having the new Sage customer number in order to authenticate the payroll process and it probably won't work if you have the old Sage customer # in the system.
Key generation (according to Sage) takes 30 minutes to update.
TL;DR:
Build in $500 as a pre-upgrade test to be sure the Sage keys actually function because on older versions like 4.3 there is a good likelihood of the keys being defective (generated based on old customer #) .