I think the Justices are well aware of the importance of the issue and will, if they do overturn any aspect of the law, will likely seek to minimize the turmoil on the country, probably by writing as narrow of an opinion as they can.
The useless answer to the question is that if part of the law is determined to be unconstitutional, then the situation will be such that it is as though that portion of the law is not, in fact, law. With a large complicated law that was crafted as an interdependent whole, I think the challenge for the Court will be determining suitable spots for severability if they do decide to strike down part of the law.
Personally I doubt they will invalidate the entire ACA.
I did consider when making my FIRE decision what I would do if the healthcare landscape changed again. I decided there would probably be a way for me to make it work in whatever new / next system we have. So I personally didn't worry about it too much.