Move CTA & Task due dates into the past

I have a number of CTAs with Tasks, where the last Task is due on the same date as the CTA due date. I can't move the CTA due date into the past easily, because I get this error: 

If I change the CTA due date into the past, I would like to get the same option that I get when moving a CTA due date into the future, so that I can slide all those CTA due dates into the past as well:

My specific use case is that we're just implementing Gainsight, and I'm trying to load in the CTAs for renewals and newly-signed customers, since I'd like those CTAs to be there for CSMs to work off of as if we'd had Gainsight all along. I'll use a Rule to generate the CTAs, but I can't use the Rule to apply the Playbooks, since I would have to manually change every Task's due date before changing the CTA due date. Instead, I will need to:

1. Have the Rule generate all the CTAs with incorrect due dates. (I can't have the Rule generate the CTAs with the correct due dates right off the bat because of a separate feature request.)
2. Then manually change every CTA due date (or use DataLoader to do so)
3. Then manually apply the appropriate Playbook to every CTA