I would:
- Download the release notes of the target upgrade OS (in this case 6.3.1.8) and read them completely. I would pay special attention to the upgrade instructions section and the upgrade caveats sections in there. There are also changes in behavior in the release notes that you might need to be aware of.
- Open a TAC case so that they can review your logs and config to see if there is anything else that you are doing specifically that needs special attention. There are many late-breaking issues that are not in the release notes that TAC would be able to tell you about, so that you can avoid issues. Aruba typically does not go back and edit release notes, so opening a TAC case could late breaking issues that you would not be aware of.
- Simulate your upgrade in a lab with your existing configuration if possible. Simulating the upgrade might reveal specific issues that you have in your network that cannot be replicated by TAC. Simulating your upgrade with similar hardware should be a standard part of considering an upgrade; even a minor one. It is also the best way to get an idea of how long each portion of the upgrade would take. If you find any unexpected issues, you have a chance to have them addressed before your upgrade.
- What is not included in any release notes is upgrade strategy. There might be a specific approach to upgrading that would help you avoid issues or make your migration easier or save a lot of time. TAC, through asking you questions and reviewing your configuration might be able to suggest these more efficient approaches.
- Ask about your upgrade on Airheads Social so that others can tell you about their experiences. Not all issues are reported to Aruba, and Aruba is unaware of them, as a result. Asking questions on Airheads Social is a great way to get as much information as possible.
To answer a specific question about your upgrade, bridge mode on RAPs should be fine.
I would in general upgrade the 3200XM to the target version of 6.3.x first and make sure your network is stable. I would then backup the flash on the 3200XM and restore it to the 7200 series. Please review the 3000 series to 7200 series lab guide here: http://community.arubanetworks.com/aruba/attachments/aruba/unified-wired-wireless-access/14802/1/7200%20Lab%20Guide_v1.1.pdf which details how to do this migration.