Does it cause an integrity check error?

Don't know yet, I had just run the change (on a single account fortunately) yesterday and discovered the issue today.  I will be running a DI on AR and BK this evening to see what gets reported then, will revert the change.  Might have to wait until the next bank rec is posted to apply the desired change.

Will cause issues with Bank Rec, because vendor/customer no longer exists that is in the Bank.

Not true

Jay - are you thinking that if the customer # change is run that the bank rec posting won't balk? Will it simply be a matter of the receipt in AR never showing as cleared? After running the number change if you tried to select the receipt to be reversed it can only be found under the old customer # so I assume the link between the 2 modules is then broken.

While the PU5 is not available, and you have the issue, you can fix it in a database level by changing manually the PAYORID field from the BKTRAND table.

Yes, probably but I prefer not to make modifications to the DB directly.  PU5 is now released and already installed in the client test environment :-)

Again, just sharing information.  There are still issues with the Customer Number Change module.  I have installed PU5 in Test and ran a test change, it failed with an error on BKJTRAND, the process did not complete successfully and there are inconstancies in AR as to which tables were successfully updated.  Working with tech support on the issues.