I'm sure you will get a number of opinions on how to do this. My suggestion will be a bit different based on whether you will be installing this directly into an OS n a server or if you will be using virtualization. Virtualization makes it easiest. I'm also an advocate of being as safe / conservative as possible. Here's my thoughts:
1) Install 2019 on a clean OS (and/or VM)
2) Perform your initial configuration work
3) Make a backup / take a snapshot
4) Run the migration
5) Do your testing
6a) If you find any issues or changes you want to make, you can always restore to one of the previous points to address those issues, re-run the migration and re-test.
6b) If you don't find any issues or necessary changes, then when you are ready to go live, simply restore back to the point just prior to the migration and then re-run the migration like before.
Thank you Eric,
yes, this could be a valid approach, assuming you should not make changes to basic data transferred by the migration, such as Statuses, Priorities, Categories, Users, etc.
Otherwise you should remember the changes you made in some way: for example exporting the data from single tables.
Maybe the right answer is simply manually remove all the Tickets before starting the new migration :-) .
Do you see any pitfalls doing that?
Have a nice day.
If you are using virtualization, I'd think the best bet would be to simply make any changes in the snapshot that is post-install and pre-migration. You could then re-run the migration from scratch on an updated copy of that snapshot so you'd see the full impact. Trying to delete out data is far more time consuming and risky path.