I'm guessing they're talking about a firmware update to address any early adopter issues. Microsoft has already promised to issue updates before the devices ship for certain issues that review units have been seeing, however. Otherwise, unless it's a major issue, Microsoft typically runs on a once-a-month software update schedule - on the second Tuesday of each month, they'll push out updates for all their stuff. So the earliest you can except this "upgrade", most likely, is July 8th. August 12th might be a more reasonable timeframe to allow for the discovery of various early adopter issues though.
This, however, operates on the assumption that there will be significant issues at launch, which isn't necessarily true - but then again, I'm an optimist who's had a fair bit of luck being an early adopter in the past.
If, however, they're talking about an upgrade of the actual hardware, it's unlikely that we'll see one until the Surface Pro 4 hits - the silent upgrade of the SP2's CPU from 4200U to 4300U was an unusual event, and also made very little difference in day-to-day usage. I suspect (pure speculation on my part) it also had to do with the fact that the SP3 would also end up shipping with the 4300U, getting Microsoft a bulk discount on parts and an early testbed for the CPU - but the Surface Pro 4 should ship with Broadwell, and Microsoft isn't just going to silently slip in a whole new CPU architecture.