Disney executives--speaking to investors--have been saying that the entire MyMagic+ system will be up and running by the end of the year. It could be as early as November 1st, as they are currently scheduled to "test" the system at all resorts by the end of October.
Once it's fully implemented, you may not have the option of using the old legacy system. They may need to keep the FP machines in the parks to service day guests still using paper tickets. But guests at Disney hotels won't have paper tickets or KTTW cards...they'll just have Magic Bands. And you can't get a paper FP ticket with a Magic Band.