Subject Design database question

I'm working on a database which now, will be used in several places,
but not as a central database, but as a seperate database at every
place. Within a few years, it is quiet possible that the seperate
databases will merge into 1 (thus a central database). I use a lot of
Autogenerated unique keys, but when the seperate databases will
merge, the key will not be unique anymore... how can I avoid this?
How can I design this database so it will be easy to merge into 1 at
a later stage, but with the uses of an autogenerated key (thus a
primary key of 1 field?)

Thanks in advance,