I have a synced version of my data on my NextCloud server. It is the only copy of my data. It was synced there from my previous Android client. I am setting up new Android and Linux clients, and I want to sync the data FROM NextCloud to my empty Joplin client.
Yes if you think about it, that would be the same as adding any other new client to your sync target.
There is also an option there to wipe all local data and force a complete download from the target.
If you are paranoid about it then there is nothing stopping you taking a copy of your entire sync folder just in case something goes wrong.
It is on the desktop client. Why don't you sync a version to a desktop client first, then at least it is easy to make a full JEX backup etc. if anything goes wrong.
Otherwise on Android you could clear out all the local data first (from within the Android app settings - not Joplin) but I thought you said these were all new clients anyway? You should just be able to set the sync target and it will download all the data.