Using a 20v Magsafe adapter will not hurt a regular MBP. The adapter for my 2010 15" MBP has a specified output of 16.5v to 18.5v, depending on load. An extra 1.5v won't have any effect. The reason for the voltage increase is probably because the standard Magsafe may have problems maintaining at least 16.5v with the increased load of the RMBP's display and larger battery.
The Magsafe to Magsafe 2 adapter is for using the RMBP with a Cinema or Thunderbold display. The displays have substantial power supplies that can manage the extra current requirements while maintaining a minimum of 16.5v.
It would be interesting to see how a standard 16.5v Magsafe adapter would hold up while supplying an RMBP at full CPU and GPU load, with a low battery. I have seen it reported here that MBPs running at high CPU load have had battery drain while being powered by the Magsafe.
The Magsafe to Magsafe 2 adapter is for using the RMBP with a Cinema or Thunderbold display. The displays have substantial power supplies that can manage the extra current requirements while maintaining a minimum of 16.5v.
It would be interesting to see how a standard 16.5v Magsafe adapter would hold up while supplying an RMBP at full CPU and GPU load, with a low battery. I have seen it reported here that MBPs running at high CPU load have had battery drain while being powered by the Magsafe.