Objection 1. It would seem that religious who are raised to the episcopate are not bound to religious observances. For it is said (XVIII, qu. i, can. Statutum) that a "canonical election loosens a monk from the yoke imposed by the rule of the monastic profession, and the holy ordination makes of a monk a bishop." Now the regular observances pertain to the yoke of the rule. Therefore religious who are appointed bishops are not bound to religious observances.
Objection 2. Further, he who ascends from a lower to a higher degree is seemingly not bound to those things which pertain to the lower degree: thus it was stated above (88, 12, ad 1) that a religious is not bound to keep the vows he made in the world. But a religious who is appointed to the episcopate ascends to something greater, as stated above (Question 84, Article 07). Therefore it would seem that a bishop is not bound to those things whereto he was bound in the state of religion.
Objection 3. Further, religious would seem to be bound above all to obedience, and to live without property of their own. But religious who are appointed bishops, are not bound to obey the superiors of their order, since they are above them; nor apparently are they bound to poverty, since according to the decree quoted above (Objection 1) "when the holy ordination has made of a monk a bishop he enjoys the right, as the lawful heir, of claiming his paternal inheritance." Moreover they are sometimes allowed to make a will. Much less therefore are they bound to other regular observances.
On the contrary, It is said in the Decretals (XVI, qu. i, can. De Monachis): "With regard to those who after long residence in a monastery attain to the order of clerics, we bid them not to lay aside their former purpose."
I answer that, As stated above (1, ad 2) the religious state pertains to perfection, as a way of tending to perfection, while the episcopal state pertains to perfection, as a professorship of perfection. Hence the religious state is compared to the episcopal state, as the school to the professorial chair, and as disposition to perfection. Now the disposition is not voided at the advent of perfection, except as regards what perchance is incompatible with perfection, whereas as to that wherein it is in accord with perfection, it is confirmed the more. Thus when the scholar has become a professor it no longer becomes him to be a listener, but it becomes him to read and meditate even more than before. Accordingly we must assert that if there be among religious observances any that instead of being an obstacle to the episcopal office, are a safeguard of perfection, such as continence, poverty, and so forth, a religious, even after he has been made a bishop, remains bound to observe these, and consequently to wear the habit of his order, which is a sign of this obligation.
On the other hand, a man is not bound to keep such religious observances as may be incompatible with the episcopal office, for instance solitude, silence, and certain severe abstinences or watchings and such as would render him bodily unable to exercise the episcopal office. For the rest he may dispense himself from them, according to the needs of his person or office, and the manner of life of those among whom he dwells, in the same way as religious superiors dispense themselves in such matters.
Reply to Objection 1. He who from being a monk becomes a bishop is loosened from the yoke of the monastic profession, not in everything, but in those that are incompatible with the episcopal office, as stated above.
Reply to Objection 2. The vows of those who are living in the world are compared to the vows of religion as the particular to the universal, as stated above (88, 12, ad 1). But the vows of religion are compared to the episcopal dignity as disposition to perfection. Now the particular is superfluous when one has the universal, whereas the disposition is still necessary when perfection has been attained.
Reply to Objection 3. It is accidental that religious who are bishops are not bound to obey the superiors of their order, because, to wit, they have ceased to be their subjects; even as those same religious superiors. Nevertheless the obligation of the vow remains virtually, so that if any person be lawfully set above them, they would be bound to obey them, inasmuch as they are bound to obey both the statutes of their rule in the way mentioned above, and their superiors if they have any.
As to property they can nowise have it. For they claim their paternal inheritance not as their own, but as due to the Church. Hence it is added (XVIII, qu. i, can. Statutum) that after he has been ordained bishop at the altar to which he is consecrated and appointed according to the holy canons, he must restore whatever he may acquire.
Nor can he make any testament at all, because he is entrusted with the sole administration of things ecclesiastical, and this ends with his death, after which a testament comes into force according to the Apostle (Hebrews 9:17). If, however, by the Pope's permission he make a will, he is not to be understood to bequeath property of his own, but we are to understand that by apostolic authority the power of his administration has been prolonged so as to remain in force after his death.
Source: Summa Theologica (New Advent)