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First, I would be recommended to export all your data into Excel so you can Import it in future to some other test management app or select another test management app and migrate all your existing data from Zephyr and once you will rest assured in terms of data then decommission the zephyr. I have done the same thing in the past.
You would need to back up all the data associated with Zephyr (tickets and test cycles). The test cycle data would be inaccessible and I would suggest porting the tickets to a different issue type that is not utilised by Zephyr. Once it is deactivated the data will not be accessible.
Hope this helps
I am curious to know that once I will port my Test issue to some other issue type then am I able to see all the steps? I guess no, I have tried in the past and I didn't the get success.
If you have some best practices for porting any test issue type to other custom issue type then please share it with us.