How to modify jhipster entity after starting development

Hi all I'm trying to develop an application using jhipster and i have some questions. Can i modify my entities after starting development i'mean if i add some new attributes to my entity and import the new jdl file will this operation delete the custom code added by me (back end and front end). if yes what is the best way to modify the entity and save my customization and the old data in the database? Thanks All