Can't insert a separate older multitrack session into another session

I created a multitrack session for a film, some months ago, and in it would have a lot of inserted audio clips already sync'd to the video. A new cut has been transfered to me and I must polish and make adjustments to the old soundtrack. I want to avoid re-inserting all the clips I added as that is tedious and, ideally, just re-sync, the tracks to the new cut while having freedom to edit the individual clips (so mixing down the old session is not an option here).

I'm trying to combine that older session with the new one but the option to insert a new session is greyed out

Under Edit tool:

The old .sesx was created within the 2024 Audition so I don't believe it's a version mismatch - even then, why would the option be unavailable for "New Multitrack Session..."

What must I do? I'm open to alternative solutions to combining/merging the two separate .sesx files.