If your 3rd party smart watch can sync data to the Fitness App/Health App on your iPhone, this data will still be included in your fitness history if you start to use an Apple Watch.
I have used a Garmin Watch before I switched to an Apple Watch, and the Garmin App synced the workouts and other health data to the Health App on the iPhone.
All the fitness data and awards from that time are still part of the Fitness App data. The Apple Watch can not show previous workouts saved in the Fitness App and the Health App on the iPhone, it will just continue to record your workouts.