The migration from wM 6.5 to 8.2 depends a lot on the components that you use . There are lots of changes that have comeup for different wM components viz. IS, Broker , TN. Take a good look at them.
If you have a typical integration environment involving IS,Broker, MWS , TN then you should not have much issues using the side-by-side approach for upgrade.
All your Portal(MWS) settings for Broker territories & gateways can be exported for upgrade purposes. IS clustering in wM 8.x uses tangosol coherance. Java & flow services are least impacted. Your Integration logic resides in packages so its a matter of JVM changing to a higher versions. If you have not used WmRoot services then you are quite upgrade-safe.
Adapters have changed less when moving up the upgrade line. JDBC v6.5 is still available with tweaks in wM 8.2 SP2. The other adapters too have incorporated tweaks in them while towing the upgrade line. Most features from previous releases are maintained whilst adding newer ones. A good round of testing is recommended for all adapter services used in the code.
If you have biz processes then you would need re-engineering as pre-wM 8.xx they weren’t BPMN 2.0 complaint.
If you have CAF then a deep testing into new 8.x environment is highly desirable. For any UI developed using Java, .NET consuming WS* that your EAI setup has exposed won’t much seek retrofitting but only good testing.
So a rule of thumb is to do a side-by-side upgrade to fix all issues in a staging environment and then move ahead with elan. If you plan to make serious changes to your existing integration architecture then you should plan accordingly.
Good luck for your upgrade.