So you bought the older Rotella T 5W-40 that also came in a blue jug? If so, don't worry about it...I read on BITOG that the new T6 formulation is still very similar but with some slight additive variation. If anything, most of the change was marketing (new look and name).
I wouldn't change it out early, that'd be a waste of perfectly good oil. Lots of us were using that first anyway because of how good it is. I'm still running it because I have a small stash of it and want to use that up first.