Posting on behalf of Anthony D.
We want to plan the upgrade of our current environment from 6.5P4 to 7.5P2. I want to know any problems with compatibility between 5.5 and 6.5/6.7 vSphere.
In order to upgrade from Zerto version 6.5u4 to 7.5u2. You will first have to upgrade to version 7.0u3 and then to version 7.5u2. There is no option to go directly from version 6.5u4 to 7.5u2
Please check our interoperability matrix to check compatibility with vSphere (6.7u2 supports all 3 versions you mentioned)