Why the heck wouldn't you ask about upgrade... " ummm, so when can I expect to nearly double my salary". Seems like a legitimate question to ask in my opinion.
If you have a quality resume, good aviation experience etc. Netjets should be courting you, not the other way around. NJA is hurting for the same quality applicants they had 6-7 years ago. You should ask anything you are curious about. NJA is not in the position to be picky concerning simple career questions.
On the other hand, anyone who goes to a NJA interview not knowing that upgrade is going to probably be 10+ years, definitely has not been paying attention or done any research. Which could be a red flag also.
Bottom line, don't be afraid to interview them also. They can not afford to be that picky. NJA is no longer the only game in town.