Well, what does 6.4 get us? The NPR upgrades are true awesome, but not that relevant for this game.
Interception movement. Convenient, but has workarounds.
New mineral generation, if you're not keeping the old minerals.
Underground infrastructure and forced labor -- not likely to be relevant.
Reduced BFC size.
Visible energy weapon impacts.
Basically, there aren't actually a whole lot of benefits to upgrading.
Plus we're rather fond of our civadmin, and our scientists aren't so bad. You could put those back in if you're willing to muck with the database, but that's adding work on more work.
I certainly won't object if the others say to upgrade, but unless you go into detail like replicating the officers, we'd want to rework a bunch of things.