IMO the value between the braces is either a GUID or a Replication ID but I'd go with GUID. Not because replication hasn't been supported since about version 2007 but because the GUID is likely a composite of several things, at least one of which ID's a users pc - possibly right down to the hard drive. That would make sense in terms of trying to keep non registered users out.
IMO, if there is a need to convert you will need to look into other methods of employing security. What those are depends on how secretive the data is, how likely is it that anyone would be curious enough to meddle and the level of Access knowledge of those people. If data is very sensitive, Access may not the right tool for the job. You can thwart 99% of your users if you do it right and that may be good enough. The most security I ever needed to employ was a back end password, front end accde with most options disabled (e.g. shortcut menus, nav pane hidden, etc.) and shift click bypass property disabled. I kept nosy people out by maintaining a table of users where one field was their Windows log in ID, so it didn't matter what pc they were on. I also didn't allow registered users in if my code detected that the application path was not as expected (to prevent take home copies for example). AFAIK, all that worked - I even had supervisors from other departments call me to tell me they couldn't get in, which not only revealed that yes, Virginia, there are nosy parkers in the organization, but also that things were working as expected.
Perhaps all you need to do is make the conversion but dispense with anything based on old Access security (part of which is the mdw file you mentioned). However, sometimes it is easier to start over than it is to remove/undo large chunks that are no longer needed; especially if security is being checked many times throughout your code project.
EDIT - BTW, no one needs to point out to me that the be password can be discovered. I'm just saying it was good enough for us, especially when the other barriers are taken into account.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.