When upgrading to a newer version you will not loose already collected data.
I recommend to read the release notes before upgrading and fix the issues if needed mentioned in the report of the assessment tool.
Verify and check the solution adapters if they are compatible and supported with version 8.0.
Before you perform the upgrade of vRealize Operations Manager, take the nodes in the cluster offline and take a snapshot of each node. If you must perform a rollback of vRealize Operations Manager to the previous state, these snapshots accelerate the rollback operation.
High level steps:
1. Run Assessment tool and review report.
2. Take cluster offline.
3. Create a snapshot of each node in the cluster
4. Install the product update using vRealize_Operations_Manager-VA-xxx.pak, in this case you need to use the vRealize Operations Manager 6.6.1, 6.7, 7.0, and 7.5 Virtual Appliance upgrade .pak file.
5. Verify if upgrade is completed successfully
6. Bring cluster online if not done already by upgrade process.
7. Remove the snapshot of each node after successful upgrade.