Great question for clarification!
From p.5 of the Compendium:
As the Doomtown story progresses, certain people will evolve and mature by virtue of their
choices and circumstances. These changes will be represented by dudes with the Experienced X keyword, where X is the experience level of the person. Different versions of a dude card are all considered one card for purposes of deck-building.
As a Noon play, you can replace your dude card in play with a different version of that dude
from your hand. The new card replaces the old, keeping all goods, spells, tokens, and markers,
and remains under any game effects. The card that was replaced goes to your discard pile.
However, you can only replace a dude card with another dude card whose experience level is no
more than one greater or lower (non-experienced dudes are considered experience level 0). Thus
you can replace a non-experienced dude with the Experienced 1 version, and vice versa. You can
replace a given dude no more than once per turn.
Replacing a dude does not trigger “enters play,” “leaves play,” or “discard” effects."
This is done as it's own Noon play as defined above, therefore since we are not Shoppin, there are no costs involved; it's a free replacement, noting the restrictions above (experience level).