Is it safe to change Apple Watch battery with more capacity? For example, while the original battery capacity for Apple Watch Series 1 (38 mm) is 205 mAh, can I replace it with a 273 mAh (38 mm) battery (originally for Apple Watch Series 2, 38 mm), and assuming if it work, would extend the standby time for Apple Watch Series 1 (38 mm) much longer than the original design? Will it work, and be safe?
It turned out that the battery of difference series of Apple Watch is not compatible with each other, due to their connector interfaces are very different. [reference] 
So, as much as safety has to be concerned, the idea won't work in the first place.