You can migrate directly from 6.5 to 8.0, however the scripts given by wM REQUIRES you to perform a 6.5 → 7.1 → 8.0.
This means you would need to get a valid 7.1 running before you can move to 8.0, no need to get 7.1 on production 
The database structure has change between versions, and while trying to reuse existing database you need to know internal structures, which you don’t.
Process for converting database requires then to execute the mirgration for the current suplied chain.
For instance 6.5 processes ran in Modeler, and stored in a repository inside WmModeler package. There is a tool for extracting this models and adapt them to new internals from 6.5 to 7.1, however not directly to 8.0. Unless you build your own tool then again the 6.5 → 7.1 → 8.0 stands.
Not to mention the new cluster settings getting rid of Repo servers and using now tangosol.
Depending what you need to migrate TN, Monitor/PRT, BAM etc the migration will be easier or harder.
The effort to build a migration tool is cost intensive, and jumping over 2 versions is not a common task, and not worth the investment.
There are many reasons why you would prefer when having 200+ IS’s and 12+ Brokers, to be done by PS. Still you can choose doing it yourselves.
You will require at least 3x the time with same resources, make your numbers and decide what will be best for you.
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