Objection 1. It seems unlawful to pass from one religious order to another, even a stricter one. For the Apostle says (Hebrews 10:25): "Not forsaking our assembly, as some are accustomed"; and a gloss observes: "Those namely who yield through fear of persecution, or who presuming on themselves withdraw from the company of sinners or of the imperfect, that they may appear to be righteous." Now those who pass from one religious order to another more perfect one would seem to do this. Therefore this is seemingly unlawful.
Objection 2. Further, the profession of monks is stricter than that of canons regular (Extra, De Statu Monach. et Canonic. Reg., cap. Quod Dei timorem). But it is unlawful for anyone to pass from the state of canon regular to the monastic state. For it is said in the Decretals (XIX, qu. iii, can. Mandamus): "We ordain and without any exception forbid any professed canon regular to become a monk, unless (which God forbid) he have fallen into public sin." Therefore it would seem unlawful for anyone to pass from one religious order to another of higher rank.
Objection 3. Further, a person is bound to fulfil what he has vowed, as long as he is able lawfully to do so; thus if a man has vowed to observe continence, he is bound, even after contracting marriage by words in the present tense, to fulfil his vow so long as the marriage is not consummated, because he can fulfil the vow by entering religion. Therefore if a person may lawfully pass from one religious order to another, he will be bound to do so if he vowed it previously while in the world. But this would seem objectionable, since in many cases it might give rise to scandal. Therefore a religious may not pass from one religious order to another stricter one.
On the contrary, It is said in the Decretals (XX, qu. iv, can. Virgines): "If sacred virgins design for the good of their soul to pass to another monastery on account of a stricter life, and decide to remain there, the holy synod allows them to do so": and the same would seem to apply to any religious. Therefore one may lawfully pass from one religious order to another.
I answer that, It is not commendable to pass from one religious order to another: both because this frequently gives scandal to those who remain; and because, other things being equal, it is easier to make progress in a religious order to which one is accustomed than in one to which one is not habituated. Hence in the Conferences of the Fathers (Coll. xiv, 5) Abbot Nesteros says: "It is best for each one that he should, according to the resolve he has made, hasten with the greatest zeal and care to reach the perfection of the work he has undertaken, and nowise forsake the profession he has chosen." And further on he adds (cap. 6) by way of reason: "For it is impossible that one and the same man should excel in all the virtues at once, since if he endeavor to practice them equally, he will of necessity, while trying to attain them all, end in acquiring none of them perfectly": because the various religious orders excel in respect of various works of virtue.
Nevertheless one may commendably pass from one religious order to another for three reasons. First, through zeal for a more perfect religious life, which excellence depends, as stated above (Question 188, Article 6), not merely on severity, but chiefly on the end to which a religious order is directed, and secondarily on the discretion whereby the observances are proportionate to the due end. Secondly, on account of a religious order falling away from the perfection it ought to have: for instance, if in a more severe religious order, the religious begin to live less strictly, it is commendable for one to pass even to a less severe religious order if the observance is better. Hence in the Conferences of the Fathers (Coll. xix, 3,5,6) Abbot John says of himself that he had passed from the solitary life, in which he was professed, to a less severe life, namely of those who lived in community, because the hermetical life had fallen into decline and laxity. Thirdly, on account of sickness or weakness, the result of which sometimes is that one is unable to keep the ordinances of a more severe religious order, though able to observe those of a less strict religion.
There is, however, a difference in these three cases. For in the first case one ought, on account of humility, to seek permission: yet this cannot be denied, provided it be certain that this other religion is more severe. "And if there be a probable doubt about this, one should ask one's superior to decide" (Extra, De Regular. et Transeunt. ad Relig., cap. Licet.). On like manner the superior's decision should be sought in the second case. On the third case it is also necessary to have a dispensation.
Reply to Objection 1. Those who pass to a stricter religious order, do so not out of presumption that they may appear righteous, but out of devotion, that they may become more righteous.
Reply to Objection 2. Religious orders whether of monks or of canons regular are destined to the works of the contemplative life. Chief among these are those which are performed in the divine mysteries, and these are the direct object of the orders of canons regular, the members of which are essentially religious clerics. On the other hand, monastic religious are not essentially clerics, according to the Decretals (XVI, qu. i, cap. Alia causa). Hence although monastic orders are more severe, it would be lawful, supposing the members to be lay monks, to pass from the monastic order to an order of canons regular, according to the statement of Jerome (Ep. cxxv, ad Rustic. Monach.): "So live in the monastery as to deserve to become a cleric"; but not conversely, as expressed in the Decretal quoted (XIX, qu. iii). If, however, the monks be clerics devoting themselves to the sacred ministry, they have this in common with canons regular coupled with greater severity, and consequently it will be lawful to pass from an order of canons regular to a monastic order, provided withal that one seek the superior's permission (XIX, qu. iii; cap. Statuimus).
Reply to Objection 3. The solemn vow whereby a person is bound to a less strict order, is more binding than the simple vow whereby a person is bound to a stricter order. For if after taking a simple vow a person were to be married, his marriage would not be invalid, as it would be after his taking a solemn vow. Consequently a person who is professed in a less severe order is not bound to fulfil a simple vow he has taken on entering a more severe order.
Source: Summa Theologica (New Advent)