Salve Chuck,
the regular way, serve the years the peregrini were inscribed (regular 20-26 years) there wouldnt be a chance to earn the citizienship and change to legion. Already the age of the men would provide it.
In some cases, several auxiliares or complete units earned the citizienship for their well done work.
That wouldnt ment, that they can change to unit as easy, but another kind of recompence the troops was to bring them to another unit.
That can mean another unit in the ranking of the troops itself (very intersting article to that in "Studien zu den Militärgrenzen Roms"), which means under different cohorts, legions and the inner of the legions (centuria and cohortes).
It may thinkable that an auxiliar with the right of a roman would be done in that way.
But i really dont know a case, what dont mean that there isnt one out there 8perhaps someone in here knows one case), but i never recognized one.
For more informations you can read also Domaszewski book, "Die Rangordnung im römischen Heer" which was new published in the late 60s and overworked.
And perhaps Alföldis "Die Hilfstruppen in germania inferior" will help a bit.
I know, all these is german, but out of that i just know some french works, which wont be more helpfull anyway