MATLAB: Am I unable to I edit a MATLAB 5.3.x (R11.x) GUI using GUIDE in MATLAB 6.x (R12.x)

editguiguideMATLABmodifyoldr11r12

I have several GUIs which I created in MATLAB 5.3 (R11), and since I now have installed R12, I would like to be able to implement and edit them there. The problem is that when I try to invoke an R11 GUI while in R12, and then use the GUIDE command like I did in R11, I am not able to edit the old GUI.

guide(gcf)