Then of course, there's the array of locally-converted uniforms used in the AWI...
And to say nothing of the field modifications made during the SYW in North America which were completely different again! Basically, every British commander appointed to North America from George Howe to his brother William, to John Burgoyne and Charles Cornwallis had completely different ideas about how (if at all) to adapt to the local environment.
I don't do the French and Indian War currently, but George Howe's changes to organisation and equipment are very interesting and show that - even though they were well aware of the difficulties in North America - successive British commanders seem to have had to relearn the lessons that the elder Howe had before his death.
So 1768 as a clear barrier, that's easy to remember
The artist Don Troiani has suggested that it might be less clear than previously thought. His painting of the Boston Massacre, for example, shows grenadiers still wearing their old cloth mitres - as the new fur ones might not yet have been introduced! IIRC he argues that older uniforms were modified to fit the new pattern until they were completely unserviceable and replaced. You see the same kind of thing later when soldiers cut down the previous uniform issue into summer fatigues.
There's also the neverending debate about whether or not some British regiments actually wore their official uniforms in the field. E.g., did fusiliers actually wear fusilier caps in North America? Grenadiers reputedly stopped wearing their own fur caps at some point halfway through the war.