The research on the Sanaa psalimpsests is still ongoing. I often feel there is no point in debating this a lot. The majority here is convinced the Quran is some divine scripture, sent down from heaven by an angel. Rational arguments often fail to convince those who have taken that leap of faith.
The most convincing text manipulation is the adding of "and the nazarenes" to the polemic aimed at the rabbinical jews. The text manipulation can be detected by the breaking of the rythm (cantillation) of the verses. Next to it, it renders the quran to contradiction. In one verse it claims the Nazarenes are most closely related to the believers, on the other hand it forbids the believers to take Nazarenes (together with Jews) as allies and pictures them as enemies.
There is the change to "?ar-rūḥ al-qudus? dropping the "al" to make it fit the later discourse of revelation by Gabriel, whereas with the "al" it can only denote the christian concept of "the Holy Ghost". This text manipulation is linked to late introduction of the myth of the night journey.
Changes to Q 2:89 and Q 2:101 to again, grant divine origins to Quran and the Rasul.
Compare:

(From verse 144, Sura 3 in folio 6v (Paris BNF arabe 328a)
vs

(From the Samarkand folio)
All of these are documented at
http://rootsofislamtruehistory.com/subpages/Codicology_and_suspected_verses.htm by EM Gallez.
There are many other examples. I'm not a scholar, these topics are actively being researched by academics way more knowledgeable than I am. They not only document these differences but also explain the motives for those manipulations.
@Roshan: if you are referring to the 2 pages of the birmingham quran of which only the parchment has been carbon dated, it hasn't changed my opinion.