Originally Posted by
emcampbe
iim also not sure if other circumstances affected my (in-)ability to change directly, if anything, would be upgrading to C+ using DL.com using SlyPesos. So that could have caused an issue.
This is probably the case.
I ran into this liquidating my Amex Platinum credit by using "Pay with miles" for $50 of a flight to trigger the additional collection. The resulting eCredit could only be re-applied to a new flight by an agent because the website itself can't apply eCredits derived from mixed Skymiles and cash payments.