Can I enable milestones for requests that were created before milestone configuration was completed?

Milestone generation starts from the point the milestone configuration was completed. If the catalyst for creating a milestone was before the milestone configuration being complete, there's no way to go back in time and create those milestones.

Whether any milestones are applied to these in-flight Requests depends on the milestone's Start Condition.

For example, if a First Response milestone is supposed to be created when the Request was created, but the milestone creation was completed after the Request already existed, then you'll not have the First Response milestone for that Request.

However, if you also created a milestone to Confirm Closure when the Request is put in a Waiting status, and this milestone was configured immediately after the Request was created, then the new Confirm Closure milestone will be added if the Request's status is changed to Waiting.