You do not need to have the upgrades in your account to request the upgrade, nor do you need the upgrade credits in your account for the upgrade to clear. However, you cannot check in your companion for the upgraded segment(s) unless you have the minimum number of upgrade credits in your account to cover the upgrade(s). So, I would suggest that you purchase the credits once you have a pretty good sense that the upgrades will clear. Keep in mind that if you buy online or over the phone, the upgrades can take a few hours to appear in your account. So don't leave it for the day of departure!