He did not change the Mosaic law - he fulfilled the Mosaic Law at the the Cross. Until His death on the Cross, the Mosaic Law was still in force - with His death, the law was fulfilled, the veil of the Temple was torn, and man was no longer under the law but under grace.
But at the Last Supper, they were still under the Law. You really need to study these things more...
The Mosaic Law was passing away DURING the Last Supper. The passing away of the Old Covenant was COMPLETED when Jesus says so on the Cross.
Jesus knew that he was going to die, and how. He SAYS he is instituting a sacrament of the New Covenant. Notice that JESUS SAYS “Take this and drink. This is the chalice of my blood of the NEW COVENANT.” See? NEW COVENANT. The night BEFORE the Crucifixion.
You really should try reading the Bible sometime. In it, Jesus says a lot of interesting things.