Is it possible to book the XTRA seat (a MileSaaver AAward) in a different cabin (e.g. F) than the one purchased (i.e. Y)?

Would this present any problems for int'l flights (e.g. transpac), especially for flights originating in a different country? Has anyone tried this before?

There is no C inventory for a PVG-ORD flight, but plenty of Z inventory. Would like to purchase the 'xtra' seat as an 'upgrade' from Y to F.