iCloud sync of Documents

I just transferred all my data from a 2015 macbook pro onto a 2019 model. both models are now signed in on my Apple ID and both have Document and Desktop Folders checked in iCloud Drive. And I've already revised a Word document in the new 2019 model...

Q: if I now turn on my 2015 model, will iCloud Drive attempt to sync my docs from that model and accidentally revert the doc I just revised back to its former version?

Q: should I sign out of Apple ID on the 2015 model, to avoid any possible syncing issues? I don't really plan to use the 2015 model again unless there are issues with the 2019.

Q: can anyone give me a good link for advice on how to properly erase the drive on my 2015 model?

Thanks,

Mike