If it can change on its own, I would just blame the computer gremlins and change it back.
Just kidding, but for real, the reference to the dll should work with the full path or with just the name of the dll, though it is recommended to avoid the full path in case the location is slightly different on one machine compared to another.
I just glanced at ours...
In External References, we have 'Microsoft.Office.Interop.Outlook.dll'.
And in Namespace Imports, we have 'Microsoft.Office.Interop.Outlook'.
Dave Morris, 3Ci at Southern Company