I was looking at an itinerary where premium economy long haul would be just over $100 more each way or about $300 round trip compared to the regular economy fare that I currently have booked.

After doing some math, it looks like if I do premium economy both ways, I'll re-qualify for Star Alliance Gold while if I stay in economy, I'll be a bit short and need to do a mileage run. Due to my travel plans, that mileage run will likely need to be a US domestic flight on UA. I'm considering doing an upgrade at the check in desk so I was wondering a few things:

• Will I be getting the premium economy fare in this situation? There's no point doing the upgrade if I'm not getting the additional miles.
• Will the price to upgrade be the same as the difference in fare currently shown on the website? Or will upgrades be cheaper than that given that they have loads of inventory?
• If I upgrade just the first leg of the flight but not the return flight, will the fare class of the return flight change in any way?
• I currently have three seats to myself both ways in economy class on the SQ flight and my understanding is that in premium economy, the armrests can't be lifted. Will that ironically make Premium Economy less comfortable than regular Economy?