Marcus, this is confusing because it runs counter to the official upgrade [instructions](http://There is no need to uninstall ZBrush 2018. You can have both 2018 and 2019 on the same system.), which state that it’s critical to deactivate zb2018 in order to avoid problems. 2018 no longer functions if you follow these directions. What people are asking, I think, is if there’s a way to reactivate 2018 after upgrading, in order to keep them both active at once.
Some people claim to have upgraded successfully without deactivating 2018 first, but it’s clear there’s a potential for problems if you don’t. You can always roll the dice, but I wouldn’t want to contact support for any problems arising if I hadn’t actually followed the official install directions.