To be sure I'd call the manufacturer. Have the lot numbers from the can handy when you do. I would not recommend mixing old and new on the same job.
I can speak from experience that Thompson's Water Seal was reformulated 5 or 6 years ago and the sheen of the finish and clarity of the product are different between the old product and the new one. They performed the same in the end, but half of my driveway looked strange for a good six months (that was the reformulated one - it is milky and doesn't soak consistently - they may have fixed that by now but I don't know).
For that matter, I've had Minwax change the color of one of their stains - that one was a real pain in the ***. While their support staff gave good mixing instructions to make the old color it did not make my project go smoothly, nor did they offer to compensate me at all. Why they didn't change the name or number I will never understand...
I now call the manufacturer with all old product/new product questions.
