Question on restore

Dear Experts,

We have a major application change next week.We are going to take rman cold backup before the change.In case if we had a situation to restore:

1)Do we need to delete existing data before restoring the cold backup?

2)In case if we use flashback database,will flashback database depends on undo retention?we are setting up flashback recovery area size and flashback retention target for 5 days but we have undo retention set to 5 hours.Just want to confirm if flashback database has any dependency on undo retention?

Thank You