This is the upgrade path I got it from your documentation when upgrading from very old versions of Bamboo.
5.14+ → 6.5 → LATEST
|Add-ons||Latest available Add-ons-V||Compatible Bamboo-V|
|Atlassian REST API Browser||3.2.2||6.7.3|
|Hung Build Killer for Bamboo||3.0.15||6.3.4|
|Plan Runners for Bamboo||3.6.5||6.10.3|
|Chef cookbook tests||1.11||6.8.3|
|Variable tasks for Bamboo||1.1.24||6.9.2|
|Build Times for Bamboo||1.1.9||6.7.3|
|Zephyr Enterprise for Bamboo||1.4.1||6.6.3|
Welcome to the Community!
***Edit*** My bad, I went back and looked and we actually upgraded to 6.5.1 ahead of upgrading to 6.9.2.
Looking at the release notes & the upgrade notes, I believe there are some larger changes from a DB migration perspective that require the intermediate upgrade. So in that case I would recommend upgrading to 6.5.x ahead of going to 6.10.3
**Also** if you look at the 6.4.x upgrade notes, you will see this note:
"Bamboo 6.4 is shipped with a native mechanism for monitoring builds. For that reason, the Hung Build Killer plugins becomes deprecated."
As for app compatibility, you need to decide if those apps are mandatory to have running upon upgrading to the newer version of Bamboo, in which case you will want to make a request to the app creators in the marketplace to update their apps to be compatible with the latest version. If you can live without any or all of them then you can upgrade and disable the incompatible apps until they are updated to support the 6.10.3 version.
You should start with an upgrade to Bamboo 6.5.x, all of your existing apps are supported on that version. From there read my comments about app compatibility.
I hope that helps!