If I upgrade the Brocade 7500 to FOS v6.4.3f3 will that be compatible with the attached Brocade 5100/5300s being at FOS v7.3.0b (LSAN config)? And should I upgrade the 7500 first?
Sorry, I meant to reply to your original post. First... yes all will be compatible in an LSAN configuration. My personal recommendation, would be to upgrade the switches first, 7500's last. Reason being, the tunnels will go down and you will want to schedule a maintenance window for this to occur as traffic will be impacted.
On a side note, I will put up announcements on the Brocade Community homepage announcing the patch firmware(s) availability, so please keep an eye out.