I recently signed up for Qobuz and have kept Tidal. If I cancelled Tidal would all my Tidal albums be replaced by their Qobuz equivalent? Or would I have to add them manually?
I did this recently, I used the Soundiiz service. You have to pay a subscription but once I did it and was satisfied with the result I just cancelled the subscription.
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Soundiz is the way to go.
Couple of days after I subscribed to Qobuz they emailed me a link / coupon for Soundiz.
Pulled over my Tidal collection and Discogs vinyl collection into Qobuz.
Easy peasy. 
If you logout from Tidal (in ROON), your library will default to whatever Qobuz albums you added (regardless if they are set as primary or not). If you added the Tidal only version of an album, that album will not be automatically replaced by his Qobuz version, though that will be neat, unfortunately interface wise it’s very hard to get Roon to do neat stuff…
If you don’t logout, but you canceled your TIDAL subscription, i think nothing will change, but you’ll not be able to play the Tidal albums (they will still show up in your library, because Tidal doesn’t delete your account when you cancel the subscription).
If you want to transfer (or keep everything synchronized) between streaming services (a lot of them, including Qobuz and Tidal) then indeed Soundiiz is your best friend (as others have already said).
However, be warned that none of your Roon internal stuff (playlists, favorited album tracks, taggs, etc.) that you had based on your Tidal collection will be replaced by the Qobuz equivalent!
There are already some topics going in that direction, e.g. here: https://community.roonlabs.com/t/portability-across-music-sources-tidal-qobuz-etc/
I recently switched from Tidal to Qobuz… I had to add the Qobuz albums 
I simply went to soundiiz.com and moved all Tidal albums to Qobuz. Over 1000. Took 10 minutes. Roon picked up all the additions in Qobuz. Tidal account closed.