If installing 2019 as an upgrade to 2018, there are some issues. The biggest is that some preferences can be set only in 2018 (32 bit) and then 2019 integrates them after you do so. If you have files on an external drive, this is easy to test my changing the drive name in Finder. Easy to reset the path in 32bit—can't be done in 64 bit on my system.
I plan to do a clean install of 2019 when the hard drive arrives. If this still occurs, I may remove 2018 completely, then reinstall 2019.
Before installing 2019, I duplicated, then changed the name of my BIAB 2018 folder by adding 2018 to the name—hopefully, this will allow me to keep my 2018 installation intact while still doing a clean install of 2019.
Again, easy enough to test.
I also installed 2019 as an upgrade to a previous 2018 installation.
The installer did not overwrite my existing 2018 installation, but renamed my existing 2018 application to "Band-in-a-Box-32bit.app" and left it sitting next to the new version.
So both versions can peacefully coexist on the same system.
As Mike said there may still be some shared preferences between the two versions, but I think most of that has been worked out.
Personally I would not delete the old version of the 2018 app, until I was sure how 2019 gets along on your machine, especially if you are in the middle of any projects.
It is still a completely new release, so you may encounter some small glitches or something that does not work as expected on your particular system.
I'm sure that any existing problems will be fixed by PG Music, but in the meantime the old application only takes up less than 80 Megs of disk space, so there really isn't any compelling reason to be in a rush to delete it.
Of course this is only my opinion, so you can take it for what it's worth.
Hopefully this helps.
- Jay