A few admissions on the leavened Eucharist:
The requirement of unleavened bread is for liceity. In the early centuries, both Eastern and Western Churches used leavened bread for the Eucharist, but in the eighth and ninth centuries the use of unleavened bread became the general custom in the West. In keeping with the scope of the Code, the canon properly addresses only the practice of the Latin Rite… (“The Code of Canon Law, Text and Commentary,” page 659).
dioceseofgfb.org/Diocesan_%20Policies/ref_eucharistic_bread.htm
Fr. Joseph Jungman – in his book The Mass of the Roman Rite – states that:
"In the West, various ordinances appeared from the ninth century on, all demanding the exclusive use of unleavened bread for the Eucharist. A growing solicitude for the Blessed Sacrament and a desire to employ only the best and whitest bread, along with various scriptural considerations – all favored this development.
“Still, the new custom did not come into exclusive vogue until the middle of the eleventh century. Particularly in Rome it was not universally accepted till after the general infiltration of various usages from the North” [Joseph Jungman, The Mass of the Roman Rite, volume II, pages 33-34]
Fr. Jungman goes on to say that, “. . . the opinion put forward by J. Mabillon, Dissertatio de pane eucharistia, in his answer to the Jesuit J. Sirmond, Disquisitio de azymo, namely, that in the West it was always the practice to use only unleavened bread, is no longer tenable” [Jungman, The Mass of the Roman Rite, volume II, page 33]
"Now, the fact that the West changed its practice and began using unleavened bread in the 8th and 9th century – instead of the traditional leavened bread – is confirmed by the research of Fr. William O’Shea, who noted that along with various other innovative practices from Northern Europe, the use of unleavened bread began to infiltrate into the Roman liturgy at the end of the first millennium, because as he put it, “Another change introduced into the Roman Rite in France and Germany at the time * was the use of unleavened bread and of thin white wafers or hosts instead of the loaves of leavened bread used hitherto” [Fr. William O’Shea, The Worship of the Church, page 128].
"Moreover, this change in Western liturgical practice was also noted by Dr. Johannes H. Emminghaus in his book, The Eucharist: Essence, Form, Celebration, because as he said: “The Eucharistic bread has been unleavened in the Latin rite since the 8th century – that is, it is prepared simply from flour and water, without the addition of leaven or yeast. . . . in the first millennium of the Church’s history, both in East and West, the bread normally used for the Eucharist was ordinary ‘daily bread,’ that is, leavened bread, and the Eastern Church uses it still today; for the most part, they strictly forbid the use of unleavened bread. The Latin Church, by contrast, has not considered this question very important.” [Dr. Johannes H. Emminghaus, The Eucharist: Essence, Form, Celebration, page 162]
“Thus, with the foregoing information in mind, it is clear that the use of leavened bread by the Eastern Churches represents the ancient practice of the undivided Church, while the use of unleavened bread by the Western Church was an innovation introduced near the end of the first millennium.”*
) in the East, though it’s not licit in the West (it’s still valid, and you can do it in an emergency).
