I will never see a converted 3D re-release of a 2D film, on principle.
The only remotely good things that might come out of this are:
1.) LFL will probably have to do new scans of the OT in at least 4K in order to convert them to 3D;
2.) The original footage that was altered with CGI may need to be rescanned and recomposited from scratch for 3D (though alternately, they might just convert them using the digital files from '97/'04?);
3.) In order to convert any of the original visual effects to 3D, all of the original VistaVision effects elements will most likely have to be re-scanned, restored and recomposited;
4.) The 3D re-release may cause a serious fan backlash, and there is a (very slim) chance that Lucas and co. might end up being shamed and embarrassed into finally releasing the OUT in HD;
5.) If #4 happens, #1-#3 may ensure that there are high-resolution raw digital scans of the OT that can be used as the starting point for a restored/remastered HD OUT.
Preservation-wise, some good MIGHT come out of this whole 3D thing. (Emphasis on "might"; this is still Lucasfilm we're talking about, after all...)