I've already set a mailer to work with a campaign, can I still modify this mailer?

YES - Even if you are using a mailer in a campaign, it is still possible to modify the mailer and save changes. These changes will be incorporated immediately in the campaign. It is recommended that you first Deactivate the campaign and then make and save changes to the desired mailer.

A few important things to note:

  1. You cannot change the subject line of a mailer that is being used in a campaign, even if it is deactive. This is a system limitation. The workaround for this is to temporarily change the selected mailer within the Send mailer step in the campaign to a different mailer and Save changes. Then make your subject line change to the original mailer and then reselect it in Send mailer step and resave the campaign again.
  2. If you change the contents of the mailer while the system is currently sending messages, some of the messages prepared prior to your changes may content the old mailer content. To prevent this from happening, do not make changes to an active campaign with targets in the affected 'Send mailer' status.?