A duplicate's AC changes as your Dexterity changes
The spell's description doesn't explicitly say one way or the other whether the AC is calculated at the time of the spell's casting or whether it changes real time, which means we need to apply logic to get to the bottom of this one.
AC is defined in the PHB (pg. 14):
Your Armor Class (AC) represents how well your character avoids being wounded in battle.
Furthermore, the mirror image spell description says (PHB, pg. 260):
... the duplicates move with you and mimic your actions, shifting position so it's impossible to track which image is real.
These two statements and the fact that the duplicates' AC is derived from your Dexterity implies that the duplicates must be as dexterous as you are, and that they only "dodge" an attack because you were capable of doing so (i.e. if the attack was aimed at the "real" you, you would have been able to dodge it).
If your Dexterity changes during the duration of the spell, then you are more/less dexterous now than when you cast the spell, and thus better/worse at dodging attacks. Given that the duplicates "move with you and mimic your actions", it stands to reason that they must now be as dexterous as you have become, since they must still be capable of dodging what you can (or can't) now dodge.