Short answer is you can't.
What am I even doing here?
As the title suggests the answer is to update to either 8.0 or 8.1 sliverlight and for the love of god store your ids locally.
In Windows Phone 7 there was a property we could use called ANID
this gave you a user specific id based on the logged in Microsoft account (across devices).
Then in Windows Phone 8 ANID was phased out and instead we were given ANID2
which is a more specific id based on the publisher id of the application.
This was done to improve privacy so no entity could track a user across devices and applications.
EVERYTHING IS AWESOME!
Wait, didn't you say you can't upgrade?
Microsoft.Phone.Info.UserExtendedProperties so no access to ANID or ANID2. So we need to stay on Silverlight for now.
This is where I got to before cursing the world and asking myself why I didn't store the id locally. Store your ids locally!
Don't worry if you are as well, here is how to fix it.
If you are truncating the ANID with Substring(2, 32) then all you need to do is append
to the start and
"&E=f48&W=3" to the end and the converter will work for it.
The conversion can also be done in C# with a few lines of code: