If you sell your houses, your population decreases.
You will not lose existing fields you have, but you will not be able to get new ones until your population numbers increase.
Population number also impacts community buildings, factories and land expansions. If you didn't build all of them, you will need to build houses again.
Bottom line, if you didn't build all available cbs and factories, you will likely need to rebuild your houses. If you did finish all town development, nothing bad will happen if you sell your buildings 😉