I believe the watch backup will be in your iPhone backup in iCloud so when you get the new iPhone, you just set it up using your iPhone backup data and your Watch backup will be transferred to the new phone.
Just did it last night with a new iPhone 6. I had to do a reset on my watch first to get it to pair with he new iPhone. I first loaded my new phone with the backup from my old phone ( you could also do this from the cloud if your current phone is backed up to the cloud). After resetting the watch I started the pairing process and was given the option to restore the watch with saved settings--option shows up on your phone during the pairing process in the Watch app. Hope this helps.