Is convertible the same as interchangeable?

Yes and No.  Signature Convertible Circular needles allow you to convert the overall cable length within a size of Signature Circular Knitting Needles.  The new product line launched in May 2013.  Unlike other interchageable needles Signature Convertible Circulars can only change cable lengths within 1 size of needle stalk.  For example a US 3 needle stalk can only be coverted to different lenghths with US 3-24", 32", 40", 47" size specific cables.  A US 4-24" cable for example would not work with a US 3 needl stalk.