Just double-checking... When you switch to a new watch, you keep all your activity data, right? I have a streak going, and I don't want to mess it up... :-D
If you've made an iCloud backup then you should be able to restore the backup of your current Watch and associated data to your new one. In regard to the health and activity data, it is backed up via iPhone if you do an encrypted back up via iTunes. So that data is always available to you.
Safest bet would be to do a backup of your phone, then once that's done you'll want to un-pair your current Apple Watch (it backs up during the un-pairing). When you setup the new one it'll give you the option to restore from the latest Apple Watch backup.