re-joining
I'm not aware of any "new" policy.
E-mail to the Cabin Crew Service Center and ask for a rejoining form. Send it back and they will contact you. The process takes 3-4 months.
Hopefully, you got "recommended for re-joining" on your file when you resigned.
I think you can keep your old grade if you re-join in 1 year but you loose your staff number.