I am working on a short-term project that was developed on Archetype 10/13 on AEM6.2, because the maven version mentioned in the main POM is an old versioned one. The Classic UI components are supposed to be upgraded to Touch UI, so that it supports for AEM 6.5 too going forward. In the process of upgrade, I have updated the POM file with the required versions of the respective dependencies and that had been successful upon code build. But when I tried upgrading classic ui components and writing few backend services/servlets, I started getting the error "Unable to load compiled class " which I believe is related to the SCR Annotations(even after the dependency version was upgraded I see this error). Due to this, I was wondering if upgrading the project to AEM 6.5 Archetype 23 help? I wanted to see if I can move the codebase to Archetype 23 or is there anything that I might be missing for fixing the error - "Unable to load compiled class".
Migrating from AEM 6.2 in Classic UI to AEM 6.5 in Touch UI is going to be a BIG project. I'm speaking from experience as a project I'm working on from AEM 6.3 in Touch UI to AEM 6.5 in Touch UI is going to take roughly 300 man days.
You would have to share the whole stacktrace with us so we could help you identify the source of the error but if you've already narrowed it down to annotation then you probably have an incompatibility between WCM Use API and Apache Felix annotations vs Sling Models and OSGI annotations.
If I were you I would:
Notify the people in charge that migrating from 6.2 to 6.5 is not a walk in the park
Start a fresh project from Archetype 23
Start porting your code from your old project to the new project, one class at a time and testing to see if
It works on the server
Your unit tests still run (they probably will not).