AFAIK there is no transition exam, you have to directly take the Platform Developer I exam.
The Salesforce Certification website (http://certification.salesforce.com/platform-developers) states that
The Salesforce Certified Platform Developer I credential is designed
for those who have knowledge, skills, and experience building custom
declarative and programmatic applications on the Force.com platform.
So you have experience building declarative applications, since you are DEV401 certified ;)
Now focus on the programmatic side (but do not forget the declarative side so far).
Carefully read the study guide (http://certification.salesforce.com/SG_CertifiedPlatformDeveloperI.pdf?v=8) as there are more topics than in the DEV401 exam.
You are expected to know Apex and Visualforce, SOQL, and even Lightning Components.
Think of this exam as if it was DEV501, without the programming assignment.
Hope this helps.