Hi
To minimise set up time, follow the instructions here to unpair your old Apple Watch (which will include syncing your latest Activity data to your iPhone) and then pair your new Apple Watch, including selecting to Restore from Backup when setting up your new Apple Watch:
Be sure to unpair via the Watch app whilst your devices are connected via Bluetooth. In addition to erasing the old Apple Watch, this will remove Activation Lock, leaving it ready for a new user to set up as their own.
If preferred, you can keep both watches paired to your iPhone, switching between them as and when you wish. Activity data will sync between them automatically as you do so (note that only one Apple Watch can be active at any given time):
Use more than one Apple Watch with your iPhone - Apple Support