UPDATE - 02/02/2018 - uploaded the headmorph that I have made for Flemeth. Here are the mods required for the morph to work:
Dalishious' 45 Real Skin Colours
ViLiSSa's Skins Special Pack 1
Jenya's Random Hairstyles
Dalishious' Chasind Face Paint

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Very simply, changes Flemeth into the Chasind robes in both encounters - i.e. the first time you see her during/after Ostagar and when you confront her on behalf of Morrigan.
(She wears the similar Tevinter robes in her appearance in Morrigan's fade nightmare, so I didn't bother changing that appearance)

I just didn't like her in the commoner clothes or the mage robes, and I haven't seen this on the nexus yet. If this mod already exists somewhere, then sorry 'bout that.

Installation:
Just extract the archive into your override folder.

Uninstallation:
Delete the mod files from your override folder.

Conflicts:
Will conflict with any other mods that change Flemeth's utc files, such as Improved Atmosphere. To avoid any conflicts with mods such as these, make sure to delete the conflicting files from the other mods.

You can also place my files in the toolsetexport folder to prioritise it above other mods. If you don't have a toolsetexport, you can just create one yourself. Any mods placed in here will override others of the same kind.

No other mods are required for this to work.

Other mods shown:
More covered female chasind robes retexture by zwierzomods
I think the vanilla design for the Chasind robes are absolutely awful, so I highly recommend downloading this as well (if you haven't already)
Enchanter Robes Mod v7 by Dadriell
This mod edits the mesh of the Chasind and Tevinter (?) robes to have hoods, which I much prefer. Unfortunately, it isn't automatically compatible with the retexture above - I did a personal mash-up to make the two work together. If you know anything about texture modding, it's super easy to make.
Covered Enchanter Robes by me
I've now gotten permission to release my mod that makes the re-texture compatible with the mesh - it can be found here: https://www.nexusmods.com/dragonage/mods/4861
Practical Morrigan Robe by magpiedragon
This is the nice robe that Morrigan is wearing.