New battery showing thermometer when attempting to charge

While I can’t say for sure as I am not fully familiar with the way the Apple Watch operates, I would assume that icon is indicating that some temperature sensor in the watch is reading out of the expected range, likely due to a fault. As other Apple devices have temperature sensors in the batteries, I would expect the Apple Watch to have one as well and given that is all that should have been changed, I would guess you got a faulty battery that is reporting an incorrect temperature reading. As a result, the watch will not power on in typical Apple fashion. My first step would be trying another battery so unfortunately it looks like you will have to open the watch up again.