I'm using a '49mm-52mm Step-Up Ring' and a '52mm Macro Reverse Adapter Ring for Pentax K'  to reverse mount my [SMC PENTAX-A 1:2 50mm][1] lens which has a 49mm filter thread. It's working great and I don't see any disadvantage. Of course it would be cheaper and simpler to use a '49mm Macro Reverse Adapter' but I was unable to find one in the local store (many years ago).



  [1]: http://kmp.bdimitrov.de/lenses/primes/normal/A50f2.html