Not sure whether it'd have to be two modules, there also could be two flavors of the dialect in the same module. We'd use the right one by default, based on the current MongoDB version.

Regarding testing, at least the TCK should be run for both flavors. I guess that'd require some changes to the testing infrastructure, e.g. our test runners, or maybe something around parameterized tests, not sure. In a second step there could be tests which assert specific interactions with the datastore based on its abilities. But that may become a bit hairy, also not sure whether it's worth the effort.

But as said, I don't think we need to address this for final, as the current dialect should work with 2.6 as well, it only doesn't take advantage of the new capabilities there.