GSquared,

I was going to say that I had disabled a trigger as part of the change script but you made me go and look again and turns out there's one more trigger that I didn't actually get to disable. Looking into it, it's copying data into history table which I didn't update and the reason it was working for column A & B is because that trigger wasn't moving those columns, only the older column C.

When I saw that implicit conversion error, I got fixated on the fact that I needed to have a GO between the ALTER and UPDATE to ensure that the changes becomes "visible" which I did originally forget in my first draft.

Mystery solved with egg on my face.

Thank you!