I'm less sure of the formal rules in this case. In your first change, you were making a change to the destination airport which is definitely going to force a reprice. Buying up tends to get a lenient interpretation from most agents, but it does feel to me that doing this would reissue the ticket and nominally require a re-validation of inventory. Unfortunately (or fortunately as the case may be), the bottom line is that a sufficiently motivated agent can create or break an awful lot of rules for you, and consistency at that level of technicality is pretty rare. Given that you are allowed to do things like book a confirmed ticket in X and waitlist an upgrade to I on a space-available basis, I think the biggest outlier in your example is that fact that it was an upgrade.