Airwave 7.5.x will work with AOS 6.3, however you will be missing fields, dashboards, and descriptions on devices that Airwave 7.7.x will bring forward.
If I make the assumption here you are using Airwave in read-only mode. in this mode, newer devices, like AP-225, will likely show up as 'unknown' or have unpredictable descriptions. Bad news of course. My recommendation is to have Airwave and AOS version 'match' current levels for maximum compatibility.
If you are using Airwave in Read-Write mode, it won't work well as some new settings (VHT for dot11ac as an example) would be missing. Again, recommendation, upgrade if possible.
Upgrading Airwave is typically a straightforward endeavour... within 1-2 hours max the system is ready to go. Is there a specific limitation where the customer wants to stay with 7.5 ?
JF