I don't have the latest version yet but I ran into this issue just now when removing tje old version of Excel2016 and adding the newer version to the catalog. Here were my steps, which i've done on about 10 other updates this morning without issues:
1) Open Old Package
2) Unselect Unstable, Testing, Stable Catalogs and Manifests
3) Save Package
4) Open New Package
5) Select Unstable, Testing, Stable Catalogs and Manifests
6) Save Package
When I did this, I encountered the "unable to add to unstable catalog" error, but it was sucessfully added to Testing and Stable.
How I fixed it was to go into datastore viewer and delete the unstable catalog, then add Excel2016 to the Unstable Catalog. This worked and regenerated the unstable catalog fully