Do I just stick them in, or do I have to do anything at the panel such as re-registering the sensors?

Panel is an SW-1070.

Edited 12 hours ago12 hr by Eugene's DIY Den

That's what I did, no issues. I didn't replace them all as only one sensor was complaining.

Does an internal sounder also notify the panel when its battery is low? Its battery status isn't listed in RF devices.

We change battery routinely so never seen a sounder low batt.

No re-registering needed. The sensor keeps its ID, so it is just a battery swap. The only catch is that opening the cover trips the sensor tamper, so either do it in engineer mode or expect a tamper alarm that you clear with your code once the lid is back on. If a tamper fault keeps showing after that, the lid or tamper switch usually isn't seated properly.

On the sounder, rather than waiting for a warning, if you have engineer access have a look at the RF device status in the devices menu. On most SecureWave panels it shows signal strength and battery level for each wireless device, so you can see which ones are actually getting low and change them in one go.