It is not controversial among anyone that positive laws may be amended or even abolished (indeed, this happens frequently, and great saints are often responsible for abolishing the laws of previous great saints).  What you've quoted is just the preliminary remark of a bull which is concerned quite squarely and narrowly with the constitution of religious orders.  Different concessions and restrictions, over time, become more or less applicable than when they were first devised-- such is the very nature of any temporal law, so of course it'll likely be subject to change in the future when the conditions in which it was first written are no longer relevant.
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Of course, the traditionalist argument is not that Vatican II erred on prudential matters-- which the pope is perfectly free to do.  He can suppress a religious order to some degree or another, even without good reason, because there's nothing intrinsically evil about telling some order or another to pack their bags.    What he can't do (i.e., what infallibility prevents him from doing) is publish some law or another which is intrinsically evil.  So, he can't make a law that compels all the religious orders to celebrate black masses.  He can make a law that tells some religious order to disband, and he can do it with better or worse reasons, or even no reasons at all.
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So you can't really compare the suppression of orders (certainly not in this case, given that the Fraticelli were heretics-- but even if they weren't, the point holds) or some other instance of positive legislation amended to positive legislation which, of its very nature, is intrinsically harmful to the Christian faith (like the New Mass, say).

ETA: Also "ultramontanist" is a Gallican slur-- probably not the word you want to be using, it signals the wrong allegiances.  I know it's "en vogue" now but it's very ill-considered.

I disagree. I can because Pius VI did not differentiate. The Spirituals were preachers, just like the Conventuals, and therefore would be spreading their viewpoints. Otherwise, why suppress individuals and place them on the suspicion of heresy lists?

I am very familiar with your viewpoints throughout the years, so I'm sure you'll counter with a response.

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Actually I'm not sure what you're disagreeing about.  Can you please clarify?

Evidently, neither of us read the bull as carefully as we could have, nor sought greater context elsewhere. The object of the bull is to remove penalties previously attached to the sort of commentaries allowed on Pope Nicholas III's bull Exiit, which dealt with the Franciscan constitution.
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So you understand pope Pius VI to be saying that no law of the Church can ever become more or less useful than when it was first enacted?  

Not exactly. What I'm saying is that Pius VI has stated that it is impossible for the Church to allow a discipline that is dangerous or harmful. John XXII, regardless if it's the opening statement of his bull or not, said that it was possible.

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Hi LeDeg,
I'm sorry it took me so long to get back to you.
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In both instances, one is best served by a deeper, rather than superficial level of what's being taught.  In the case of Pius VI, he is saying that the Church cannot approve of any disciplines which is intrinsically harmful.  If you are doubting whether or not this qualification is just, (i.e., if you think I'm merely adding it ad hoc), then simply draw your attention to the context:
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Denz 1533 is contra the Jansenist/Pistoian belief that the liturgy approved by the Church could abandon the principles of the liturgy.  Denz 1578 is more of the same, condemning the notion that the Spirit of God (i.e., providential infallibility) would not protect against such a scenario: of the Church's discipline itself being injurious to souls.
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John XXII, on the other hand, with his prefatory remarks, makes it abundantly clear that matters extrinsic to the discipline established have made the discipline less advisable.
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Do you see the difference between a law which is only extrinsically made less advisable and a law which, of its very nature, leads to some error or another?
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If you do not, which pope do you think is right, and why?