[AccessD] Take a formatted number and turn it into a "real" number

I just got handed a database of about 30 tables (about
half are lookups), there are about 10 tables with
varying relationships that are fairly well defined. 
There is an identical - but unconnected - version of
the database at our West Coast location.  
I have now been tasked with merging the two.  At first
I thought it would be easy because it appeared they
were using two different numbers schemes for the PK's
turns out they just formatted them to "look different"
but the matching tables use the same Autonumber -
start from 1 - field type.  So now I have to figure
out a way to merge them without messing up all the
relationships.  
Does anyone have any ideas?  Both databases are still
being used so I have to come up with a way I can do it
with the least down time possible.
TIA,
Nancy