You can resize the image in PS, of course. You can specify the new size in pixels or inches, of course, which saves the hassle of converting. I think you have realized that the aspect ratio is slightly different between the two sizes. Without seeing the poster it is difficult to know whether you can "stretch" the poster slightly without it looking weird-like. Have you tried resizing it to the new ratio? In the resize dialogue there is a setting to "constrain" (or something like that... I'm not sitting in front of PS as the moment. You want that setting turned off so you can set each side independently.
IMPORTANT: Make a duplicate of your original first so you don't accidentally save over it. It is guaranteed that someone will distract at the moment you meant to either go back, or to 'save as'.
Luck.
