Yes Marty there were two versions used at all three plants during 68 production. Agree with Royce that the exact change over date is likely a soft one and slightly different depending on plant and even engine option. But in general I think we can narrow down the change to a fairly short window. Best we can do without being there and without having all the real build dates of cars we use as examples
Though most unrestored cars only have the adhesive remnants of the paper based sticker we have a few that are still legible that I was able to find. Invite others with original examples to add to the discussion or at least data points
Let me offer the following after looking through what I have available
Looking at just small blocks I found the latest vehicle I have pictures of with the early version of the sticker was 8T03J1186xx (approx mid Nov 67 build date)
The earliest example with the later version of the sticker (element part number listed) as 8R01C1426xx (approx mid Feb 68 build date)
Now if we step back and look at other engines we have original pictures from 68 with one of the famous 135 cars (68 Dearborn built 428 CJ cars used in the initial drag program) with the early version of the sticker. So this would give that car an approx end of Dec 67/first of Jan 68 build date.
So it appears that a general range could be suggested that the change over was in Jan-Feb 68
Hope this helps