The only physical difference between the two sensors is that the 2018 version swaps out a white logo for a blue logo. (techradar.com)
But do note that the chargers are not compatible, due to the slight physical differences in the two watches. (androidcentral.com)
There are a lot of physical differences to examine more closely, but size is such a dramatic one that the authors of the new study chose to start there. (washingtonpost.com)