Nintendo is apparently concerned that the Switch's Joy-Con controllers might not be flexible enough. The console maker recently applied for a US patent on Joy-Cons with hinges that would let the top half "bend" to provide a more ergonomic grip. This would work whether or not the controllers are attached -- side sections would let you slide them on to the Switch's rails.
There don't appear to be any changes to the functionality beyond that -- the biggest change would be a flexible circuit board.
It's far from certain that Nintendo will implement this in the Switch or a future console. This is the American version of an international patent from February 2018, and the largest change so far is the Switch Lite, which fixes the controllers in place. This isn't necessarily evidence of a "pro" Switch or another revision. Nonetheless, it's evident that Nintendo is still looking for ways to refine the Joy-Con design, and it wouldn't be completely shocking to see this at some point in the future.