The need for sealed plans is just about universal anymore. No town wants to take the responsibility for "approving" anything these days. I send drawings out to engineers every week, often several times a week. In NJ (we cover NJ, PA and DE), you HAVE to get sealed plans for everything outside. I've had to get sealed plans for a set of styrofoam letters pin mounted to the cash-wrap wall in a store, inside a mall. The inspector said that if he could see if from the mall common area it counted and needed a permit/plans.
Simple solution for the steel thickness, drill a small hole (1/4" or so) in the steel poles. Do it at the top if you are worried about damaging the integrity of the pole. We do a lot of surveys/installations for the big sign companies and it's common practice. Hand holes are often cut in the bottom of steel poles to allow the wires to run down inside so a tiny hole won't damage the pole.
For the engineering seal, good engineers will allow you to install a sign that is the same size/height/etc. as was there before. Hopefully you have a relationship with your regular engineering firm. We get sealed plans weekly and so our firm knows that we don't F around. However, the engineer will want proof (preferably drawings but even photos) of what was there before. I have one engineer that actually likes old footings because he knows that they have stood the test of time. We've done this quite a few times before. Now a cantilever installation may be another matter since it's more than just the concrete in the ground. All that support steel, guy wires, etc. have been out in the elements for who knows how long.
Or dig down on one side of the footing and find out for sure how deep it is. Get your young/strong helper to go out and get some exercise. In PA is should be at least 42" deep which is the frost line in the Philly area. Your engineer (or the building inspector) will have this figure for your particular area.
Worst case, get someone with a backhoe to pull out the footings and install new. If they are renovating the building, the sign work is probably a small fraction of their spending. Of course they probably allowed 37 cents for signs since "it's only a sign".
About the code, well that often has changed since the signs were installed originally. Just call the township zoning officer and ask. Many towns now have their code on line. One of my pet peeves is when you call and they say "it's in the code, look it up". Come on, it's your job to answer zoning questions!!!
