Is it possible to convert a Master-Detail relationship to a Lookup?

From the Salesforce documentation: Considerations for Relationships

Converting Relationships

You can convert a master-detail relationship to a lookup relationship as long as no roll-up summary fields exist on the master object.

You can convert a lookup relationship to a master-detail relationship, but only if the lookup field in all records contains a value.

Yes, this is a known issue. Mark Pond is correct, our information in that unit on Trailhead is inaccurate. You can convert as long as there are no RSF fields. https://help.salesforce.com/HTViewHelpDoc?id=relationships_considerations.htm&language=en_US

We've got the fix teed up and ready to go in our next release.