If you are in combat and an effect increases or decreases your Dexterity enough to change the modifier, does your AC also immediately change to use the new modifier?
Your armor class would certainly respond dynamically to changes in any value that is used to calculate AC.
Your armor falls off? AC changes. Drop your shield? AC changes.
Sprain your foot and lose Dexterity? Yup, your AC is recalculated from your new, lower Dex modifier.
AC is calculated on a case by case basis, with the player having the choice to take the best result as applicable to them.
For example, if your Dex is 17 and benefiting from both Mage Armor and Barkskin, your AC would be 16. This is because of the following:
- Mage Armor calculates AC as 13+Dex Mod (+3 here), so your AC is 16.
- Barkskin calculates AC as 16 and it cannot be lower.
If an effect were to increase your Dex to 18, then your Dex mod would change and you would use the AC calculation for Mage Armor, which will provide you the more beneficial AC of 17.
Conversely, were your Dex lowered due to some effect, you would use the AC calculation for Barkskin, which sets the AC at a static 16.