Of course it was a supper in the perspective of Passover. It had the sorts of the Passover celebration, the breaking of bread, and the drinking of the cup, but it was not the actual Passover seder because it took place on Thursday evening, not Friday evening. The lams were not killed yet, according to St. John, the death of Christ took place the next day, Friday, while the lambs were being sacrificed in the Temple.
But all this is not the point. The connection with Passover is not the meal, but the sacrifice. The lambs being slaughtered in the Temple are of the Old Covenant, the Lamb being sacrificed on the cross is the New Covenant in Jesus Christ, the fulfillment of the Law and the Prophets.
The Orthodox Church even goes on to teach the Last Supper is not the essence of the Eucharist. The full essence of the Eucharist is not our reenactment of a meal, but our participation in the cross, the tomb, the resurrection on the third day, the ascension into heaven, the sitting at the right hand, and the second and glorious coming. Eternal actions.
I guess what I am saying is that the old Passover was changed into a new Passover, a NEW PASCHA), (not a sader turned into the last supper).
On Easter we sing in church: Today Christ the Redeemer has been revealed to us as a Pascha, a sacred Pascha, a new and holy Pascha, a mystical Pascha, a most honorable Pascha, a blameless Pascha, a great Pascha, a Pascha for the faithful, a Pascha which has opened for us the gates of Paradise, a Pascha which sanctifies all the faithful.