To solve this, OI recommend to take full data and system configuration backups, install a new instance of SPS, and restore back the configuration and data on newly installed system. This will result in having a fresh SPS without the legacy compatibility issues. If you are running a virtual appliance, the whole procedure will be safer as you can benefit from taking snapshots your appliance and revert back if anything goes sideways.
In a nutshell, the steps you need to take are following:
1. Make sure you have all your data and system configuration backed up. It is still strongly advised to double check all connection policies to have valid backups. Please follow the administration guide and configure all necessary backups.
2. Stop the old SPS
3. Install and start new VM with SPS on the same version
4. Follow the restore procedure described in SPS Administration Guide
5. Make sure all data is restored and available
6. Make sure the appliance works exactly as before
7. Proceed with upgrade to 6LTS