change of employer/petitioner

[angel82673]

my sister has an approved imigrant visa petition, waiting for a visa number (she is a nurse). she is looking into changing her employer-petitioner because her current one is experiencing some stability problems. there is one employer that is willing to petition her. if she decides to change, is there a possibility to keep her priority date from the previous application?
anyone who can help to give us info will be appreciated!