In Q & A order:
1. Isn't it allowed to switch the track from classic to continuous with the same serial?
Response: No, it's not allowed. They are two different application builds ( Classic build and Continuous build ) and they require different serial numbers.
2. Why does the wizard keeps failing to create a new offline exception while it worked for an older release of both (Acrobat DC and Wizard)
Response: Because the Continuous app build that you are attempting to get the grant the offline exception for with the Adobe Customization Wizard DC is being fed the wrong (a.k.a. Classic build version ) serial number type for a Continuous build version.
3. How to solve this issue?
Response: In order to get an offline exception for the Continuous build you will need to get a Continuous build serial number and then follow the process for getting it with the Adobe Customization Wizard DC.