We are running Ovirt3.8 version in our prod, we want to upgrade to 3.8 to 4.2. 

At present we have 2 (hyp A & hyp B)physical host ovirt 3.8 is running with self hosted engine with data domain and iso one cluster and data center. What we were thing do this migration in following fashion.

Plan A.

1. Attach export domain present infra.

2. Take backup all of running vm as export.

3. Detach export domain

4.  Detach hyp B from cluster and data center

5. Install fresh ovirt 4.2 iso on hyp B

6. Install self hosted engine 

7. Attach Data Domain 

8. Attach Export Domain which was use in ovirt 3.8

9. Import all vm's from there to new ovirt 4.2

10. Once all the vm's got up and running poweroff hyp A

11. Install fresh ovirt 4.2 iso on hyp A and attach cluster.

Plan B.