It will not evaporate over time. Really the only way to lower it is to do partial drains and refills which are a real pain and waste a lot of water. Much better to just make sure that your CYA doesn't get too high in the first place.

Some pools do lose their CYA over the winter. It biodegrades and causes a HUGE chlorine demand to get rid of all of the byproducts left behind. It isn't fun when this happens and takes a TON of bleach to clear it up. Unfortunately, there is no real way to know which pools will have this happen. Hopefully, you won't ever have to deal with it.