Cripes that's screwy. Looks like you have fine access from the top, just take a corner loose from the top, then you should be able to peel the face back as you go, getting each bolt loose. Once you have the top and one side free, you can usually just unhook the other side and bottom, but if it's old enough it won't want to let go. Impact and deep well 7/16" are your friends here.
Save the old face, lay it over the new face, and make where it is folder for clip placement on the new face!
You may want to beef up the welds at the top of that pole. Typically, the angle is resting on the pipe, so the weld only helps stabilize everything. Here, the weld is holding up the
sign, looks like they figured on a 4" larger pipe when building it.