Yes you are correct that you will get online resources for 6 3 to 6.5 beacuse of oak and repo restructuring.
AEM 6.4 -> AEM 6.5 should be quick one without any blockers/impact as no major changes done on Sites.
Major changes are on AEM Assets new features, Forms new feature, SPA (multi site management feature).
Only change done on sites side:
Up to AEM 6.4, you needed to create a Content Fragment Model which is converted into the content fragment. This content fragment was placed on AEM pages using Sling Model to export in JSON format. AEM 6.5 simplifies the process.