On part two the link with the Scottish Health Education Group started in the mid-eighties (you might remember the "Motherwell, Motherwell, Go for Good Health and be a Winner as Well" abomination) so there was health related stuff up across the ground from them on.
I can remember, or at least have seen images showing firstly an advert on the roof that something along the lines of "half way down Robertson Street" on. The first SHEG advert that I can remember had something like "Go for Good Health and Get Fit for Life" on the roof. That was on it when the East Stand opened in 1991. "How Much Do you Cough Up for Cigarettes" was on it a few years after that.
After the Cooper was built the Main Stand and East Stand both got new facias for a more consistent look across four stands so on the basis that the Cooper opened in 1995 I'd say the change of signage would have come in the aftermath of that. When exactly I couldn't say.