The surnames Menzies and Mackenzie were never meant to be pronounced with a z sound.

The original spellings used the medieval yetter yogh, which looked like this: ȝ

Yogh represented a y sound in both English and Scots.

So the name Mackenȝie was actually pronounced like Mackenyie. The ȝ got replaced with a z because it looked similar, and was easier to print.

The names were still pronounced with a y sound by those in the know.

But not everyone was in the know.

When others encountered these spellings with z, they did what seemed like the sensible thing: they pronounced them with a z sound.

Hence the modern pronunciation of Mackenzie: with a z sound.

The case of Menȝies is a bit more complicated. Traditionally, it's pronounced like Mingus.

It likely came from a Norman French name Mesnières, which ended up sounding like Menyers in Scots, hence the use of yogh: Menȝies.

Eventually, the ny sound in Menȝies became an ng, which isn't far away in the mouth. The old pronunciation of Menzies as Mingus is still heard, but it's competing with the version said with a z.

The language changed — albeit in a small way — simply because printers didn't have a letter.