Since, as Paul indicated, you cannot use the workflow you describe above... you could always, with some extra work, rename the AG within your AP. But... you need to be careful and rename every instance of it!
So, I would start by opening your AP within a Pro session. Then,
- Right-click your Structure Junction table and, under Data Design, choose Subtypes.
- In the Subtypes Group of the Subtypes ribbon, click Create/Manage.
- Change the AG name.
- Then... open all the B_* tables, and for each of them,
- Search for where Asset Group is equal to 'Manhole Channel'.
- Use a Calculate on the AG field to change it to 'Junction box'.
Not all tables will have an AG field, and for those with the AG field (some times From AG or To AG) not all of them will make a reference to your 'Manhole Channel'.
Then, if you have data in your AP, you need to check the contents of the Structure Junction table, as well as the C_Associations.
I hope this helps.