Hopefully someone will have time to answer my stupid questions. I am having troubles wrapping my mind around the upgrading from 3.7 to 3.9h.
So far I understood that:
After uploading the new version to a separate database and separate folder under my actual store, I have to compare the original 3.7 to my modified one with Winmerge for example.

After I put the files I modified in the override system of my 3.7 version (didn't use it and how do I regret!), I compare my modified files to 3.9.h in winmerge. For every different files I either paste/copy the different language files and modify the code I did modified in the my 3.7.

I upload the 3.9 modified in my demo folder with the zen install folder.

I copy the 2 configure files from my old store in 3.9.

And then I install through my browser, clicking upgrade instead of install.

My problem: I have an issue with bluehost. When I try to install to try out the new version the server tells me it cannot access the new database (and I can remote access it, but not the zencart installer). It get stuck with an error too many connections. I contacted bluehost and they reset the server (can't do it myself with this host and the command flush host or reset in the sql query). But on my end it still doesn't work! So the guy at bluehost installed the new version, it was working but not anymore.

I can't understand what the problem is with my new database. I probably made a big mistake somewhere as I am not familiar with sql and phpmyadmin etc.

So my main question would be: once I have changed everything for 3.9 can I upload it in the new folder and connect to my first database (not the new one)? When I change the configure files for the new database it doesn't connect and nothing works.

I am pretty stuck right now. I would really appreciate someone's help. Sorry for the long post and in advance thank you!!