With great tedium and difficulty, in my experience.

One way would involve the following steps:

- Pick one database as the Source and the other as the Target
- BACK THEM BOTH UP!
- Determine the highest autonumber in the master table in Target
- Open Source, and delete ALL relationships from the master table to
the related tables
(You did BACK THEM UP, right!?)
- Change the datatype of the primary key from Autonumber to Long
Integer (you must delete the relationships to do so)
- Reestablish all the relationships, and check the Cascade Updates
property of the relationship. If there are "grandchild" tables, set
Cascade Updates there too - on *all* instances where the master ID is
used as a link.
- Run an Update query updating the ID's in the master table to their
current value plus a constant large enough that there will be no
overlap with any records in Target.
- Go to lunch and relax while the many updates occur
- Check your data
- Open Target; use File... Get External Data... Link to link to all
the tables in Source (Access will alias them by adding "1" to the
name)
- Back up both databases AGAIN (in a separate location)
- Run a whole bunch of Append queries to append data from the linked
Source tables to the Target tables; these will need to be run in the
correct order to ensure relational integrity isn't violated
- Groan, swear, and complain about the fact that there is data in
Source which causes duplicates or violates validation rules in Target.
Fix these errors. Run append queries again.
- Check the results

                 John W. Vinson[MVP]