Hi Mark,
Depending on exactly how you set it up, it could be pretty simple or it could be a bit more work.
If you're not trying to do a deep customization on the JavaScript, you should be able to just point your JS to the new distfiles and have that work fine - the underlying architecture change won't stop that, and we put a ton of work into making sure that the old "drop in your javascript" approach would continue to function. That said, if you want to move your project more into ES2016 land and use something like webpack, there might be a bit more work... you can see an example of how one of our Yetinauts, Brett Mason, tackled this linked from this github issue: github.com/zurb/foundation-sites/issues/10199
On the SCSS side, we worked hard to maintain backwards compatibility, so if you want to keep using the older grids you can, simply leave your @imports the same as they were. If you are ready to move to the XY grid (and I hope you do... it's super powerful and really opens a ton of new layout possibilities), you can change your import over to import that grid with @import foundation-xy-grid-classes; If you do, you'll need to update your templates to use the new grid classes.
If you run into questions, don't hesitate to reach out on github (chiming in on the issue linked above might be a good place to start), via the Foundation slack channel, or in the Forum. We've got a number of folks who are building wordpress themes who can help you out.