It's been over a decade since my last VS flight and after many years with BA I'm looking forward to trying out Branson's finest again.

The plan is for LHR-ATL, possibly returning ATL-JFK-LHR (same-day transfer, not a proper stopover), in PE. However, I'd like to splash some points on an upgrade to UC. It'll be a corporate booking made through Concur.

Questions:

• Will I be able to upgrade a H-bucket fare made through Concur?
  I've seen it written that since 2016 any PE fare is upgradable to UC, except Virgin Holidays'. This thread suggests TA upgrades may be possible, while stuart_f's excellent guide says H-bucket fares are upgradeable but simultaneously implies that TA tickets are not upgradeable. v-flyer.com says flat out that H fares of any type are not upgradeable. Which is it?
• If it's possible, am I personally able to pay my own additional taxes (i.e.: it won't be charged to my company)?
  I'm happy to fund my own upgrade but I know certain ticketing systems can't cope with multiple sources of revenue.
• If I do stop in NYC, am I able to upgrade only the JFK-LHR sector or will I require the points and available seats for ATL-JFK as well?
  Or will ATL-JFK even be upgraded automagically?

Thank you for any advice.