Q: Apple Watch Series 1 to Series 2

Hi

As long as you restore your new watch from a backup of your old one, you will not lose your historic activity.

• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support
• Back up your Apple Watch - Apple Support

To clarify, Series 1 does not refer to your existing watch.

It instead refers to a new Apple Watch model that has the same features as the original Apple Watch, except for a new dual core processor.

Also, if you were to keep both watches paired to the same iPhone, you could switch between them - also without losing any historic activity, including within the same day:

Use more than one Apple Watch with your iPhone - Apple Support

You're most welcome!