Unfortunately there’s no way to run the Pulp 2 to 3 migration from Katello versions past 3.18.5, as that’s the last version that includes both Pulp 2 and 3. 
If you’re able to restore from a 3.18 backup, you may be able to try again.
Unfortunately there’s no way to run the Pulp 2 to 3 migration from Katello versions past 3.18.5, as that’s the last version that includes both Pulp 2 and 3. If you’re able to restore from a 3.18 backup, you may be able to try again.