If a lead is previously added to the hard bounces list and we get an updated/new email for that lead...will they be emailed to that new email address, or does the lead stay on the hard bounce list? Thanks!
Once you update the hard bounced email address, Marketo will consider that update and will send the email . However, the email address will be still marked as "email invalid".
I would suggest, create a brand new trigger campaign for data management purpose:
Smart List: Email address is updated
Flow: Change email invalid to false
Schedule: Run it every time