I have been enjoying seeing the different options proposed. I like both of these contrasting pairs:
chu'wI' / ngo'wI' - newcomer / old-timer (proposed by De'vID)
taghwI' / taHwI' - beginner / survivor (proposed by loghaD; seconded by voragh)
I like chu'wI'/ngo'wI' for its simplicity and clarity, and its suitability to just about any context; I like taghwI'/taHwI' for the same reasons, and also because it is a minimal pair demonstrating the contrast between gh and H, and really emphasizes proper speech, as voragh mentioned. However, that could also be a source of confusion, e.g. when listening to a poor quality recording. The words are also indistinguishable from each other when whispered.
I think if the situation is appropriate, SIQwI' “endurer” might be good. Perhaps it might even be appropriate in a general sense. It can also take aspect suffixes to indicate whether the experience endured is complete (QI' SIQpu'wI' - one who has served in the military and is no longer in active duty; a common use of the word “veteran”) or actively occurring (Quj'a' SIQtaHwI' - one who is experienced in a sport and still participating in it; another common use of “veteran”).
In response to this note from Quvar:
PS: veteran in the military sense might be {SuvwI' po'} or {SuvwI' qan}.
tlhIngan SuvwI'pu' qan tu'lu'be'.
chu'Ha'wI'? An un-newcomer?