Never introduce a cluster member FI at a different version.
Best approach would be to downgrade the new FI to the same version first. Next promote the Standalone FI to primary, then introduce the secondary. (Though this process should not be disruptive, I would schedule downtime regardless to be safe.
Primary/Secondary roles are not chosen by the higher FW level.
If you have no need for the 2.0 features or bug fixes, then stay put where you are. If it's not broken, don't fix it.