Really need to first insulate the upstairs the best you can. If already finished, there are solutions like cutting sections of the drywall out, blowing in insulation and then putting the pieces of gypsum back up and then tape & mud, so it never appeared you did anything. Also, sealing leaky windows, doors, closing storms during heating & cooling also helps, along with keeping drapes closed. Ceiling fans in the bedrooms also help.
Post your temp from the closes vent from the air handler, then from the farthest. If icing, could be as simple as cleaning the crap out of the outside unit and the indoor coils, before looking at freon levels.
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