Yes, it's a law that says you have to replace alarms that comply with Scottish legislation. Otherwise, you have more to lose.
What will happen if I don't install it?
There are property issues involved. If you have a house fire and your home insurance company finds that you are not complying with the new law, then they may refuse to pay.
Can the batteries be replaced, or is it a new unit in 10 years' time?
Built-in 10 years Sealed lithium battery, The whole unit will need to be replaced in 10 years.