The question is whether they are right to change the rules after hiring you.
If they wanted to have it be that anyone they hired was a non-smoker and agreed not to take up smoking as long as they worked for the company well and good. But changing the rules and making them retroactive is a little dicier.
For example it is a condition of employment at my company that all new hires must have a High School Diploma or a GED.
We have a whole slew of old timers who have neither.
Would it be just for us to demand that they go back to school and get their GED or be fired?
It might be legal. But I don't think that it would be right. And companies should be seen doing what is right. It is just good business.
Companies change their own rules all the time.
I was once covered by Scotts benefits but they changed my status from permanent to temporary and then to seasonal and decided I'm no longer covered.
It sucks but it's legal.