Sorry, but the best solution to this problem may not be that quick to setup...
Workgroup PDM and Enterprise PDM can take care of this tracking for you almost automatically.
The good news is that Workgroup PDM comes with Solidworks for free. The bad news is that it really only works well for small groups...
Linking the text at whatever location you need to the custom property for the revision level could solve this problem for you.
I haven't watched these videos, I'm about to head out of the office, but I believe they may help you solve your problem:
It was not until you mentioned Pack and Go that I realized you are changing filenames, before that I thought you were just talking about a custom property. """"change the mark on all drawings from A to D""""" Are you talking about a Revision letter?
It is generally not a good practice to put a variable into your filenames, because changing filenames can break links (though not in the case of drawings).
Anyway, if you show a screenshot that may help - I don't fully understand your issue.
If you're changing filenames in bulk, you can usually do it with one command in DOS, but as Brian said, it will break the links between parts, assemblies, and drawings if you do it.