Move less than you think
Customers and companies: move them all. Open and pending tickets: move them. Closed historical tickets: leave them in the old system as a read-only archive for a year.
Teams that insist on migrating five years of closed tickets spend most of the project on it and gain almost nothing.
Run in parallel, briefly
Two weeks with both systems live, new work in the new system only. Longer than that and people drift back; shorter and you discover configuration gaps under pressure.
Rebuild configuration, do not port it
Categories, statuses and automation accumulate cruft. A migration is the one moment you can delete the eleven categories nobody uses. Start from what you would design today.
Train on the queue, not the features
Agents need to know where work comes from, how ownership works, and how to escalate. Feature training can wait until they have a reason to care.
Keep the customer-facing address
Customers should notice nothing except better replies. Same address, same portal URL if possible, and a redirect if not.