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

Converting a relationship from lookup to master-detail or vice versa can cause existing custom reports to become unusable due to the different standard report types available for each type of relationship. We recommend that you test your custom reports immediately after converting the relationship type. If you revert your relationship back to the original type, the reports are restored and become usable again.

Converting relationships from lookup to master-detail, or from master-detail to lookup behaves the same as for two-object master-detail relationships. That is, the two linked objects in the detail-subdetail1, or subdetail1-subdetail2 relationship have the same conversion limits as the master-detail relationship.

You can’t delete a custom object if it is on the master side of a master-detail relationship. If you delete a custom object that is on the detail side of a master-detail relationship, the relationship is converted to a lookup relationship.

The second master-detail relationship you create on your junction object becomes the secondary relationship. If you delete the primary master-detail relationship or convert it to a lookup relationship, the secondary master object becomes primary.

https://help.salesforce.com/apex/HTViewHelpDoc?id=relationships_considerations.htm&language=en_US (https://help.salesforce.com/apex/HTViewHelpDoc?id=relationships_considerations.htm&language=en_US)


Thanks
Rupal Kumar
Mirketa Software Pvt Ltd
http://mirketa.com/index.html