The issue I see with this is if you are using publisher and there were so type of changes to the QVPR database. If you have a backup of the QVPR from before the upgrade it shouldn't be an issues.
1. Stop all Qlik services
2. Remove the current version.
3. delete all Qlik folders.
4. install SR2 (at this point I don't it is supported anymore) and setup everything.
5. Stop all Qlik services and delete all files in the QVPR folder and then copy all files from backup
6. Restart services.
The main Draw back in 11.20 SR8 Calander object is not worked properly.FYI please go through the below links