During character creation (cf. p. 117 f. of the Core Rulebook), the player has to decide whether their character shall be
a) a young officer,
b) an experienced officer, or
c) a veteran officer.
This choice puts the following constraints on rank: Characters that are chosen to be young officers can only be ranked Ensign or Lt. junior grade. Characters that are chosen to be veteran officers can only be ranked Lt. Cmdr. or higher. A character that is an experienced officer does not have constraints regarding their rank; thus, characters from Ensign to Captain can be ‘experienced officers’.
This choice also has the following effect on talents:
a) Young officers receive the untapped potential talent. This is mandatory.
b) Experienced officers receive a talent they choose freely.
c) Veteran officers receive the veteran talent. This is mandatory.
These talents, again, have the following constraints on rank:
Young officers cannot be promoted beyond Lt. jg. while they possess this talent. This is explicitly stated in the description of said talent on p. 118. Thus, in order to be promoted, they have to first (or at the same time) exchange their untapped potential talent for another talent. Note that this change is irreversable as the talent is explicitly only available during character creation.
Since the rules explicitly state that there cannot be a promotion to Lt (or beyond) while the character possess this talent, exchanging the talent does not automatically come with a promotion. this is also consistent with the concept of career as presented in step 5 of the character creation.
Veteran Officers have to be at least Lt. Cmdr. While the talent description states “The character has a rank of at least Lieutenant Commander.” (p. 118), I would argue that acquiring the talent in play (possible at GM’s discretion) does not come with an automatic promotion. The rank of Lt. Cmdr. has to be seen as a prerequisite, as stated in the rank’s description on p. 128.
An experienced officer can obtain the veteran talent any time with a milestone, provided they have
a) a rank of at least Lt. Cmdr., and
b) GM’s consent.
While it is not explicitly stated in the rules, I see no reason, why a veteran officer should not be able to exchange their veteran talent. This would not change their rank, at all. Yet, this could be seen as dramatically inappropriate, unless the character had a radical, groundbreaking change of character and/or their relation to Starfleet.
Another thing that is not explicitly stated in the rules: I would allow veteran officers that are demoted to Lt. keeping their veteran officer talent. However, such a demotion could be a sound reason to exchange the talent (see above). I would, in this case, not allow them to re-take the talent until they regained the rank of Lt. Cmdr.
Summary: Step 5 of the character creation (lifepath) system gives your character a value and a talent. By default, both are chosen freely and this option is called ‘experienced officer’. One can freely chose to obtain either the untapped potential or veteran talent, both of which put certain constraints on rank. Yet, according to the rules, there is no automatic change of rank when either of these talents are exchanged via a milestone, later.