So, we could have high floor service?
Sure, but UPX is more than high floor, The platform lengths are 3 cars long, and there is a branding and standard for the service level that must be adhered to. Have you been to the UPX at Weston or Bloor? They have their own buildings, entrances, kiosks, fare machines, etc etc etc.
You can well say "well dont do that then" but thats not a decision for you to make, as well as the fact of the unknown of the electric GO trains.
I will say this, if the electric GO trains are high platform, it will be a complete mess trying to make Union and every GO station they stop at both high and low platform. I really hope they are not, or at least Dual platform like CalTrain. But they really should be low platform, thats the best option.
But considering Metrolinx has done some wonderful work like making the Union station ceiling for the renovations too low to accommodate overhead wiring for electrification, I would not give out high hopes of them transitioning from a UPX station to a GO-RER one in an efficient or forward thinking manner.