My understanding is that, originally, the laeti were roman citizens that were brought back by the roman army from the Barbarians that enslaved them. Then, they were given lands as coloni. But after that, laeti are considered Barbarians (laeti gentiles) that got the same status than the previous roman laeti. What I don't understand is what happened to the previous laeti, and why the Barbarians got this status.
Hmmm, I think you're simplifying this stuff a little bit. I hesitate to designate the Laeti as full Roman citizens. They originally were either the descendants of prisoners or communities of people that had surrendered to Rome. They originally settled lands in Gaul (under strict supervision of the state btw). So they're more like "Roman Citizens" with limited rights and forced military service.
Now I wonder what you mean by "same status." You mean status equivalent to full Roman citizenship? If so, I believe there's an easy explanation for why these barbarian laeti got that status and that has to do with the Edict of Caracalla. It's a pretty famous edict, it gave full Roman citizenship to a very large group of freemen throughout the entire Roman Empire.
Now as for what your question in regards to what happened to the previous laeti. Well, they're still there. They just have more rights and they're pretty important to defending the north western frontiers. The application of the term has just changed slightly based upon what's happening in the later empire.
C. R. Whittaker. Frontiers of the Roman Empire: a social and economic study. Baltimore: Johns Hopkins University Press, 1994.
G. E. M. De Ste Croix. The Class Struggle in the Ancient Greek World: From the Archaic Age to the Arab Conquests. Ithaca: Cornell University Press, 1989.