Differntial back up: what happens to existing data? Ok, this is a question borne purely from paranoia.  But i want to ask all the same.

If I make changes to a db, and then apply a differential back up at a later date, what happens to the changes I have made?

So:

1: Take full back up.

2: Take diff.

3: Change data in the DB.

4: Apply diff to db.

EDIT:

Basically I'm copying a db, and I'm wondering if its quicker for me to reprocess some data, rather than wait for a diff.  But if the diff arrives before the processing finishes, what would happen to the data I have reprocessed when the diff is applied.

EDIT 2:

The reason for wondering is that we are moving data centers.  So i have one currently live environment, and am in the process of moving live to a new location.  We have large DBs though that are taking days to move, and im wondering if i just re process the data (days of processing).  If the diff arrives before the reprocessing is finished, i was wondering what the result of applying the diff would be (rollback to when the diff was taken?), or if we would have to restore both the full and then apply the diff, making reprocessing pointless.

(My gut tells me it will roll back to the when the diff was taken, but i want to really make sure this is right).

Cheers.
Since you'd have to apply the differential backup's previous full backup, the end result is identical to if you'd just taken a full backup, made changes, then restored the backup you'd just taken - any changes made would be lost.

There are, however, tools and techniques which would allow you to perform partial restores, so you could limit the application of a differential backup to just objects which had not been modified by your changes.

Since you can't apply a differential to a database that has been recovered ( as in, restored from a full backup without the NO_RECOVERY option or otherwise WITH RECOVERY'd thereafter ) and you can't modify a database that is offline ( in this case, in recovery would be the offline state in question ), trying to do so would not only negate any changes that were made, but require a complete full restore to be applied again before the newly arrived differential could be applied.

More information regarding the reason behind your copy process ( DR redundancy system, development environment, read-only reporting server, etc ) would be needed to make a recommendation on a suitable technique for your specific problem.