According to a recent report by MacRumors, Apple has some part constraints for original Apple Watches, as well as Series 1 Watches. According to an internal document, Apple support is instructed to replace eligible devices with Series 2 Apple Watches.
We would advise to seek an Apple Store, if you experience any hardware issues with your original Apple Watch or Series 1 Apple Watch.
MacRumors also states:
The document does not mention Series 2, Series 3, Series 4, or 38mm models of any kind as qualifying, nor does it specify if the parts constraint is limited to aluminum, stainless steel, or other finishes. No further information is provided.
You may also check for send in Apple Support options here.
Aside from potential hardware issues, watchOS 4/5 does not run super smooth on original and Series 1 Apple Watches. So getting a Series 2 replacement can actually feel like a new experience.
Let us know if you have any issues with your older Apple Watches.