Is 1/2" plywood standard and appropriate for a roof replacement?
I spent 4 years representing a roofing company in NC.  Occasionally I have seen 3/8" plywood on a roof.  It is certainly not preferred by professionals interested in quality.  Mixing thicknesses is not preferred either, but if you use the proper underlayments and decent shingles, particularly archatectural, you aren't likely to notice.  Half inch on a roof is usually OK and very common, at least around here.  Obviously 3/4" is better.  If you can afford to replace all the plywood, it certainly wouldn't be a bad thing.