When upgrading firmware from version 6.4.x to 7, there have been some changes made regarding acceleration which could cause incompatibility issues. So, if upgrading devices in stages, it is possible there could be issues with accelerated traffic if the devices are running different firmware versions.
Various changes to WAN memory, communities, and TCP options have been made in 6.4.3, 6.4.0, and 6.4.4. Any earlier versions than this will not have the changes necessary to run acceleration properly with version 7 devices.
Please be aware that version 6.4.4 or newer should be compatible with any version of 7. It would be best to make sure that all devices are running at least version 6.4.4 before upgrading them to version 7 in stages.