There has been some healthy debate in the comments section (h/t to Rich and Julie M.) on whether the government was in its rights to amend the constitution. On the face of, nothing illegal has been committed -the text of the constitution was more or less followed (aside from some procedural irregularities - they didn't send the revision to the president for comments, etc).
The amendment process is also not in itself too exceptional - the French constitution, for example, also allows for the parliament united in a joint session to change the constitution with a 60% vote (it is much more difficult in the US).
Even some of the content of the proposed revisions is not too outlandish - as I have pointed out here before, countries like Mexico, Nicaragua, Paraguay and the Philippines elect their presidents in a one round, plurality-wins elections.
However, from a more principled stance, the revision is flawed. First, it appears that the government has amended the constitution not in the public interest, but so as to benefit private individuals. Holding just one round of elections will benefit President Kabila, as the vote will largely be a pro- or anti-Kabila vote, and in a one-round election, the opposition will split the anti-Kabila vote, making it easier for the incumbent to win. The justification that a one-round election will save money does not hold water - the country has to hold two rounds anyway so as to carry out provincial elections in February 2012. The other argument, that the country will be polarized by a two-round contest, is more defensible, but does not trump the fact that a one round election will give undue advantage to the incumbent (especially since the opposition is so divided). Also, while the 2006 election was very tense, the violence that did ensue was rarely East vs. West, but more due to local conflicts.
The speed at which the whole vote was carried out was also alarming, given that parliament was revising the founding document of the republic. On this count, the Republic of South Africa has a clause in its constitution that prevents any amendment from being voted on within 30 days of its submission to parliament.
The substance of the revisions are also questionable - some have argued that placing prosecutors under the authority of the ministry of justice violates the independence of the judiciary mandated by the constitution. I will leave this question to the constitutional experts, but in many European systems prosecutors are also appointed by ministries of justice; of course one can argue that those ministries are more accountable than the Congolese one, but at least pro forma they are not too far out in left field.
On the dissolution of provincial assemblies, however, and the removal of governors (after consultation with the council of ministers and parliament) I think they have gone too far. If a provincial assembly is democratically elected, and it then elects governors, the president should not unilaterally be able to remove these officials as he pleases. This was the debate in Pakistan over the years. The dictatorship of Zia allowed the president to dissolve provincial assemblies, a power that was controversial, suppressed by the 13th amendment and then restored by the 17th amendment under General Musharraf.
So the revision of the constitution should have been much more deliberate and thoughtful and should have taken into consideration the spirit of the law, not just the letter.
And, of course, Julie M. is right in pointing out that if MPs were bribed into passing the law, then it was illegal. But how to prove that?