Flexible Address Format in R11i and upgrading to R12

I am thinking of implementing the FAF method to ensure clean addresses for R12, since eBtax chokes on bad addresses relally bad.  One way to fix the problme is to work a quality program in R11i to clean up bad addresses.  This is best done with FAF across Customers, Suppliers and Banks.  The client has not done this, hence the quality issues. 

Has anyone had any bad experiences doing this part way through or just before an R12 upgrade, or what if any impaocts the FAF format may have on results in R12.1.3 address data cleanliness.