According to the step 6 in 2.5.9.3. Configuring and installing shared databases,

Install 3scale in the passive cluster. The APIManager CR should be identical to the one used in the previous step.

However, the process seems insufficient. Should this be a variation of backup/restore process? i.e. the databases does not need to be migrated, but ConfigMaps and Secrets need to be copied from an active 3scale to an passive 3scale. Also routes need to be restored on the passive 3scale.