Saying, This is the blood of the testament which God hath enjoined unto you.
Exodus 24:8, "Behold the blood of the covenant, which the Lord has made with you." The change accords with Christ's inauguration of the New Testament, recorded Luke 22:20, "This cup (is) the New Testament in my blood, which is shed for you;" the only Gospel in which the "is" has to be supplied, as here. Luke was Paul's companion, whence the correspondence.
Testament (note, Hebrews 9:16-17). Christ has sealed it with His blood, of which the Lord's supper is the sacramental sign. The testator was represented by the animals slain in the old dispensation. In both dispensations the inheritance was bequeathed: in the new, by One who has come in person and died: in the old, by the same one, only typically present. See Alford's excellent note.
Enjoined (me to ratify) unto (in relation to) you - me to ratify in relation to you. In the old dispensation the condition on the people's part is implied in Exodus 24:8, "(Lord ... made with you) concerning all these words." Paul omits this, as he includes the fulfillment of obedience to "all these words" as part of God's promise in the new covenant (Hebrews 8:8; Hebrews 8:10; Hebrews 8:12, whereby Christ fulfils all for our justification, and will enable us by His Spirit in us to fulfill all in our now progressive and finally complete sanctification.