The Great Vowel Shift - you are referring to Glaswegian, no doubt?

Nope, I don't think he was. He was referring to the one that took place between Middle English and Early Modern English. I was once the other half of a discussion in a club in Columb St Minor in the mid-'80s. The other half was a Glaswegian RAF NCO. His opening speech lasted nearly ten minutes and I could only identify one word, an oft-used expletive used as an intensifier.

Ceci n'est pas un seing.