Dunno if this is typical or not, but the sights that were on my 1909 Argentine barrel would turn on the barrel. They did require a good bit of force to get them to move, but they did turn, and did not appear to be soldered on. (I found this out while trying various methods of attempting to remove the barrel before borrowing a proper action wrench and barrel vise.) One thing that will happen though, is this will expose bare metal where the barrel was blued with the sights in position.
The M1908 Brazilian and 1909 Argentine receivers should be exactly the same as far as the barrel screwing in and mating up, but its most likely that the headspace will be changed from one receiver to the other.
BTW how much does Numrich list these barrels for?