I just found an article and looks like Series 0 and Series 1 displays and batteries are completely interchangeable.
Dan check this (all information from ifixit)
Apple Watch Teardown
Apple Watch - Series 1
The Apple Watch Series 1 is a revamp of the original Apple Watch, announced September 7, 2016. Most of the parts are the same as the Series 2
Andrew Goldberg's Article
"When we tore open the Series 2, we found a totally redesigned, streamlined, repair-friendly interior."
"To find out for sure, we warmed our iOpener and unsheathed our opening knife. With the display open, the verdict is … it’s a Series 0 in there."
"The construction in the new Series 1 exactly matches what we saw in our teardown of the original Apple Watch, right down to that annoying display cable bracket. That’s the exact same 0.78 Wh (205 mAh at 3.8 V) battery, right down to the Apple part number. Just to be sure, we checked compatibility with our original Apple Watch parts stock, and found that Series 0 and Series 1 displays and batteries are completely interchangeable."
Apple Watch Series 2 Teardown
Apple Watch Series 3 Teardown