Chosen Solution
First of all, Can you place a 273 mAh battery (e.g. from Apple Watch Series 2, 38m) into Apple Watch Series 1 (38mm) whose original battery is 205 mAh? will it be compatible? Secondly, If that works, will it be safe? Finally, What about generalizing the conclusion to other version of Apple Watch (or even to other product of Apple)?
The different series have different batteries which are not interchangeable as the connector interfaces are very different.
Series 1 - 205 mAh (38mm) 246 mAh (42mm)
Series 2 - 273 mAh (38mm) 334 mAh (42mm)
Series 3 - 279 mAh (38mm) 352 mAh (42mm) The space the battery sits is larger in the Series 2 & 3 watches so the battery within them is larger unlike the older Series 1 which has a thicker logic board so the space the battery is limited.
