"xmin" system column

Outside of "VACUUM FREEZE", is there any way the "xmin" column in a
relation can change, assuming of course the tuple is never updated
again?  I'm considering using this as a way to identify all tuples
modified in the same transaction (in an effort to group them
together), and am wondering if there's any way tuples from different
transactions could end up with the same xmin value.

I've tried both "VACUUM" and "VACUUM FULL" on specific tables and
neither seem to have an impact, but I haven't done extensive testing
against very large tables that have experienced lots of churn.

Any input will be greatly appreciated!

eric