Sitecore Forms and WFFM are 2 different modules that happen to handle the same functionality (forms). There is no migration path available and at the moment there is no mention of anything like that either (on the contrary - when asked the Forms team said this was nowhere on the roadmap because the modules are too different).
I have a website running on 9 at the moment with WFFM installed. As Forms is installed out-of-the-box, we have both modules available at the same time and this works perfectly. How we will proceed in the future is not yet clear - depends on what Forms will bring in the next updates. Whether it is recommended to use both in the same solution is a difficult question and might be opinion based. Forms is still missing some functionality so you might want to turn to WFFM in that case. Explaining your customer when to use whoich Forms module might be tricky in some cases...
Note however that someday WFFM will truly be dead (now it's end-of-life..). This will not happen before Forms somehow has all the functionality that WFFM has today. But I can't imagine that Sitecore would want to keep 2 teams alive for the same functionality so I assume (note that this is just my guess) that replacing WFFM completely is high on the roadmap for Forms.
But even at that point, don't expect a migration...