C17 doesn't look much different than C89. If you are used to K&R C there may be some adjustment but I would expect it to be manageable.
What might perhaps be more challenging is adjusting to the changes in compilers. They tend to optimize code more aggressively and so writing code that closely follows the rules of the language (rather than making assumptions about the underlying hardware, even valid ones) is more important today than it was back in the 80's.
What might perhaps be more challenging is adjusting to the changes in compilers. They tend to optimize code more aggressively and so writing code that closely follows the rules of the language (rather than making assumptions about the underlying hardware, even valid ones) is more important today than it was back in the 80's.