Apple watch

As long as you make a backup when you unpair your old watch and then when you pair the new watch when you're asked to setup as new or restore from a backup choose restore from backup and select the backup you made when you unpaired your old watch.

Have a read of these pages;

Unpair and erase your Apple Watch

Then;

Set up and pair Apple Watch

No you must unpair the old watch first so that you get all your present data into the backup,if you pair the new watch first then you won't have all your fitness data to put on the new watch because it hasn't been backed up.

When you unpair your watch in the watch app on the phone a backup is automatically made and uploaded to icloud,and that is the backup you use when you come to pair the new watch with your phone.