When you revert back to your old iPhone, you will be presented with the option to restore from backup, which will include all the data available on the Watch when you last unpaired it from the old iPhone.
Here's more information from the Back up your Apple Watch - Apple Support article:
Your Apple Watch data is backed up to the iPhone that it's paired with. This happens automatically as long as your iPhone and Apple Watch are near each other, so you don't need to do anything to keep your watch backed up.
When you unpair your Apple Watch from your iPhone, the watch is backed up completely to your iPhone to make sure that the latest data is saved. You can use the backed-up Apple Watch data on your iPhone if you ever need to restore your Apple Watch, or when you set up a new Apple Watch.
When you back up your iPhone to iCloud or your computer, your iPhone backup includes your Apple Watch data. This means that when you set up a new iPhone and restore it from backup, your latest Apple Watch data is also restored.
Hope this helps, and have a great day!