Transferring from religious to diocesan isn’t nearly as difficult as the other way around. the key difference lies in the fact that members of religious orders take vows (initially temporary and subsequently permanent). Religious orders generally tend to have a novitiate process which anyone entering the order has to go through before they take temporary vows (with a period of discernment and living in community generally required before this). So while a transferring diocesan priest would not need to go through the usual human and academic formation they would still need to redo their spiritual formation since the spirituality of an order is different to that of the diocesan priesthood.
In contrast, for a religious priest, the basic requirement is that he is able to find a diocese to accept him. In both cases however, not only does the order / diocese which the priest seeks in enter need to formally agree to incardinate (or accept) him, but also his former order or diocese needs to agree to excardinate (or release) him. This can be quite a laborious process depending on where the diocese / order is located.
In our diocese, last year, a priest who’d been ordained 30 years and served as pastor in several parishes went on a sabbatical and decided to join the local Franciscans. I never see him at their Formations or public Franciscan-oriented Masses, however, and I am unsure what his canonical status is. Don’t know if he’s even a novice or what. All I know is that he wears their habit now.
Are you sure about that? Perhaps you perceive a moral or ethical obligation, but there certainly isn’t a canonical obligation! Just remember that the holy priesthood existed long before any Religious order saw the light of day. I wish you would show more respect for the episcopate and presbyterate! They deserve it.
Can. 644 Superiors are not to admit to the novitiate secular clerics without consulting their proper ordinary nor those who, burdened by debts, cannot repay them.
At the abbey I’m associated with, we had a secular priest who became a monk. Alas he has since passed away. We have also had the opposite, men who were released from their vows and became diocesan priests.
This is more difficult but not impossible:
Can. 686 §1. With the consent of the council, the supreme moderator for a grave cause can grant an indult of exclaustration to a member professed by perpetual vows, but not for more than three years, and if it concerns a cleric, with the prior consent of the ordinary of the place in which he must reside. To extend an indult or to grant it for more than three years is reserved to the Holy See, or to the diocesan bishop if it concerns institutes of diocesan right.
Can. 693 If a member is a cleric, an indult is not granted before he finds a bishop who incardinates him in the diocese or at least receives him experimentally. If he is received experimentally, he is incardinated into the diocese by the law itself after five years have passed, unless the bishop has refused him.
My abbey had a diocesan priest who wanted to become a monk. So on the one hand he was already ordained and could celebrate the sacraments, but on the other hand he was a novice among the Benedictines. He ended up returning to his diocese before he took vows.
Multiple priests have told me this. Furthermore, the Church herself even states that the religious life is a higher state of perfection than the secular consecrated life/priesthood because they profess the three evangelical counsels. What’s more, because cloistered priests, monks and nuns profess solemn vows, it is the highest state of perfection. It’s not being disrespectful. It’s the truth.
You are correct that religious life is a higher state, in that it mirrors our Heavenly life. But the Church Fathers have referenced this to the monastic life. This is not necessarily true, for example, for the mendicant life ( Franciscian and Dominican)
But you are incorrect that a bishop is obligated in any way to release the priest from his promise of obedience.
In fact, quite the opposite, as noted, the Law of the Church prevents a religious superior from accepting a candidate UNLESS he has been released by the diocesan authority. This would not be the case if the bishop was compelled in anyway to release him.