Salesforce announced last year that they would be retiring Force.com credentials in favor of the new Platform App Builder and Platform Developer 1 certifications.
As part of that retirement process, anyone holding Force.com certifications needs to convert them over to App Builder or Platform Developer certifications.
After April 6, 2018, anyone who has not taken the transition exam to get the new certifications will be forced to take the full exam to be recertified.
So don’t wait. Take the transition exams now before it’s too late.