Hi Catherine.
I'm not sure if I've completely understood the issue, but I'm also aware at how complex certification issues can be to explain.
If I've understood correctly, you have a certification that has the "Recertification date" set to "Use certification completion date" along with a number of certified and non-certified learners.
You need to add some new content to the certification and it must be completed either…
before the learner recertifies and every recertification going forward
when they recertify and every recertification going forward
…depending on when their recertification date is due.
Those that need to complete the new content prior to their recertification date will be considered non-certified if they do not achieve this.
Based on the above, my suggestion would be as follows…
Edit the certification original path and recertification path to include the new modules.
This will handle the 2nd set of learners that need to complete the modules as part of their recertification. This will also handle any new learners that start the certification process.Create a one off program that contains the new content, and set the required completion date. This should then be assigned to the 1st set of learners that must complete it before their certification recertification date.
This program can then be monitored and learner certified statuses can be manually adjusted depending on it's results.
This being said, I'm not aware of the finer details of your set up, so I would highly recommend testing this with a short duration certification if at all possible, before making your changes to the live certification.