The battery will last for absolutlely ages, its made from the same stuff as the special camera batterys that do 500 pictures and such.
The exact same battery is used in computers to keep the time.
If in doubt just change it.
My car seems to have a reduced range lately, ill just change it if it fails, ive got a spare..
I think to reprogram it you hold the buttons down and turn the ignition on.
To get rid of the service light if you service your own car. you hold down the milaege counter reset button and turn the ignition on..