So, just like the thousands of others here, by the time I updated to 6.2.4 on my 11 month old Apple Watch 4, I used to have at least 30-40% batter left. Then the update and by 8:00 pm (14 hours use), I was down to 10% reserve and with each update through 6.2.6, it got worse every week.
I received the replacement yesterday and after pairing it to my phone, it wanted to update to 6.2.6 but I did not let it. It shows it is on 6.2 (factor I guess as being shipped). I charged to 88% at 1:30 pm upon arrival and at 10:30 pm there was still over 78% battery left. Less than 10% drain over 9 hours. Today, I took it off the charger at 5:30 am, did my 40 minute bike ride tracking with the watch and here at my desk at 2:22 pm (8 hours) I have 86% left. I have two active email boxes in the notifications and text. No phone calls yet today. All settings are factory with no added apps yet. I have the display on half brightness and auto activate upon arm elevation.
You decide what the problem is, but it sure seems like software / OS. I have not updated my phone to the most recent OS either but had not with the old watch and the old problems. There is no way my batter was weak or bad after 9 months and then all of a sudden no 14 hour life.
And FYI, I tried all the tricks you great people have posted and none worked for me.