WARNING!!!!
DO NOT PUT ALL 3 MERRILL .erf FOLDERS INTO YOUR OVERRIDE FOLDER!!!!!!

Because of the way that DA2 is set up (like that the game takes place over 7 years, but bioware thought that no one should age), there isn't a way to assign more than 1 character morph file, in your override folder, at a time.

So, how do you use this?

with each new act, you must manually replace the folder, with the corresponding act number (A1 Merrill, A2 Merrill, A3 Merrill), into your override folder.

(i.e. 'A1 Merrill' folder goes into your override folder during Act 1, 'A2 Merrill' into your override folder during act 2, etc. BUT FIRST YOU MUST REMOVE A1's folder from the override folder, before replacing it with A2's folder!!)

OR

you can download this amazing utility that takes care of the before mentioned (plus, if you use 'equip your party" mod, you can add in different armors, textures, etc. to change with your corresponding Act):
http://www.dragonagenexus.com/downloads/file.php?id=2867


CONFLICTS:
-more than 1 Merrill .erf file is in the override folder at any given time.
-required mods are not installed.

OTHER OPTIONs:
if you would prefer to not be changing Merrill's morph with each act, just keep the merrill.erf file, with the hairstyle that you like most, in your override directory throughout all 3 acts.


CREDITS:

honestRope for redistribution of the included mod file "natural hair" located at:
http://www.dragonagenexus.com/downloads/file.php?id=2582

ivolga, for the great idea! check out ivolga's aging morphs here:
http://www.dragonagenexus.com/modules/members/index.php?id=20417