My name is Contara from Zerto Support.
The CSP upgrade to 7.0 will be failed due to the customer is running on 6.0.
Just to confirm, the upgrade for 7.0 will not work and will sate the error being the customer site being 6.0
Brian from Zerto here.
Your statement is correct. All sites paired have to be at the most 1 version apart to be able to upgrade.
If you wish to install Zerto 7.0, you will need the customer to upgrade to at least Zerto 6.5