With SQL Server 2008, how can I roll back a transaction done some time before?
There's no way to roll back a specific transaction like that using the native tools.  There are (or at least there used to be) some third party tools which would let you do this, but it depends on a lot of things.

Your best bet will be to log the before and after values for all the rows that you change during out of band update, then if you need to roll back those changes you've got the needed values.  You might want to log the new values as well as the old values so that you can see if the values have been since changed again so that you can then figure out if you want to roll back those changes.