rollback osm2pgsql append changes

The answer is probably "with difficulty", unfortunately. If your new file contains changes and deletes then you'll need to find out what the previous versions of those objects were.

If, however your appended file only contained creates, then you can programmatically create a file that deletes those additions.

One thing you could do is:

• create a list of all relations, ways, and nodes mentioned in the file that you created in JOSM.
• download all these objects from the OSM API (e.g. with something like wget -Omysfile.osm http://api.openstreetmap.org/api/0.6/nodes?nodes=1,2,3,4)
• using --append mode again, add this data to your database, overwriting any modifications you made with current data from OSM.

This will not "go back" to whatever your data was before your modification, but instead "go forward" to whatever the current state is in OSM, but perhaps it helps?