It would depend entirely on how it was phased out. If it’s straight up now totally illegal, they’d have to shut down or change product. If it’s just becoming a restricted chemical, they’d have to attain licenses or some other form of permission and likely find new customers since I doubt you’d be able to sell to the general public.

I’m sure there are good and bad examples of how governments handled it all throughout history… Even recent history. Which country had the dumbass that basically made extrajudicial and vigilante killings of drug dealers legal? If such a turn happened for my product in such a country, I’d be closing up shop pretty fast if my product was made illegal…

Cocaine itself would likely be more under the “just shut down” side, since refining plants and stuff is a bit less transferrable of a skill/process than, say, chemistry skills and tools for cooking meth. Though that’s an assumption, because I don’t know the process! Maybe it’d be easy to switch to a different product.

There’s no fundamental requirement that I can think of to grandfather anyone in for anything when making laws.

But it may be politically-sensible to do so in some cases, since obviously if someone has made an investment in something that is banned, they are impacted more than someone who has not, and are probably going to take more issue wirh the law.