You can get the watch battery changed at any store that does that like (JC Penney) if you don't want to do it yourself as long as you don't need it to be waterproof. DH started changing mine and his after sending it to polar several times for ~$40 each time. The transmitter is easy to change if you can get it open. The old design was flawed but you can use a 1 Euro coin to open it.
I've had the RS400 for over 15 years and it is still going strong, same for my husband's RS300. Our HRMs are really old but they are still working without issues as long as the straps/transmitters are maintained. Last year, I got the A360 with the H7 transmitter. I hadn't planned on using the A360 for my routine workouts but I ended up doing it and stopped using the RS400. Side by side, they both worked within 1 beat of each other using the H7 transmitter.
The things I had to do to keep my Polar working:
-Never wash the strap in a washer no matter what Polar says. Early on, I couldn't get the RS400 to work consistently. The transmitter straps couldn't handle being washed on cold in the regular washer. I lost several straps before stopping the machine wash. I started rinsing mine in the bathroom sink after every workout and hanging it to dry. I use hand soap to clean it about 2x a week. After a workout, the strap is soaking wet but washing it with soap 2x a week is enough to keep it clean. I have to get a new strap once every 2 years.
-The transmitter becomes erratic when the battery is low but that is easy to change. On average, the old transmitters have lasted about 5 years with routine use. The transmitter will need to be replaced when it starts malfunctioning. I've had the H7 for about 9 months so I have no idea about its longevity but I haven't had a problem with it so far.
-If the watch is malfunctioning, you may have a lemon.
Polar has a new version (A370) that has improved tracking at rest. I'll probably get that when my A360 dies. I ended up liking the A360 much more than I thought I would.