New wineskins
One needs new containers that are more flexible. So too the new age Jesus inaugurates brings new practices appropriate to the changed circumstances, most notably in this context the joy of celebration rather than the sorrow of fasting.
All Christians would do well to reflect on whether their demeanor, life-style, and words convey to others, especially the unsaved, this joy of salvation and the lively presence of Jesus or whether they communicate, even unwittingly, a dour, judgmental attitude that is quicker to point out the wrongs of others. We must also consider, even as the message of the gospel remains unchanged, whether the methods of evangelism, preaching, church growth, music, and worship, once effective in different circumstances, have turned counterproductive and need to be replaced by new methods that will more effectively win and minister to the current generation.
Perhaps Matthew’s point is that the new does not need to be constrained by the old, and that only in this way can that which is of value from the old be preserved.