You'll just be doing a standard firewall upgrade/migration. As @jvalentine already noted you won't be able to put a 5050 and a 5250 in an HA configuration. You could simply use the migration tool, or you could simply lift the configuration from the XML files and built it out from there.
the configuration can be migrated from 5050 to 5250 as you said by XML files. But what about the active sessions in the active PAN? if no HA will be between the new 5250 Passivve and 5050 Active, thats mean the traffic will be down when making the 5250 passive to active and replace 5050 active to the other 5250.
You will likely have downtime when replacing your firewalls, you are not able to really make this transition without loosing connection for a little bit.
Now with the proper setup you actually wouldn't have to lose connection. That really depends on what your network looks like.
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