I recently changed the battery in my HRM-Pro for the first time. I'm familiar with the process since I've owned the Run and still have the Tri version of this style HRM.
This battery change didn't go so great though. Initially it was fine but next workout it strated with a 200 plus heart rate. I opened it back up and there was a drop of water on the battery and I inspected the oring gasket and it is flattened in some area, probably not sealing well.
Can you buy the Oring gasket for these? Seems like that would help with the issue although I've never had to replace it before on a HRM from Garmin.