I've always used the correct year's program for doing amended returns, but I know at the top of the 1040X you can choose which year its for...so it would seem like you could use 2018 for a different years amended return, but I don't understand how it would be using the correct tax rules or tables...I've always wondered how this works!

You would use 2017 software to prepare the amendment, BUT if the state or federal X form has changed, then you MIGHT go to 2018 to prepare the 1040X and equivalent state form. 

Generally a "lightly" modified 1040x is acceptable. I believe that would be the case for 2017.