I have recently taught myself the bare bone essentials of changing the default outfit for any actor. I'm having a problem, however. I'll use Aela the Huntress and Astrid as examples.

For Aela, I created a new outfit, and set this as her default outfit. Everything seemed to work fine; she was wearing the new outfit all the time. Then, after I drank the blood under the Skyforge (with Skjor), and I woke up in the wilderness with Aela, I noticed that she was wearing her old clothes again. I checked the CK and the default outfit remained what I had changed it to, but for some reason she's wearing her old stuff and my new outfit isn't even in her inventory. Despite this, she still has the new weapons I gave her.

Same thing happened with Astrid. When she was sitting in the shack she was wearing the new outfit I had set, but once I got to the Dark Brotherhood Sanctuary for the first time, she was wearing the Shrouded Armour.

I noticed that the Astrid actor in the CK that I modified (not the charred one) has her death item set to the "shack key". This tells me that this is the "version" of Astrid that is in the shack during the start of the DB chain. I believe the Astrid in the Sanctuary is somehow a different "version" but I cannot find it in the CK. I believe this also applies to Aela.

Does anyone have a solution?

It's not a different version of Aela: Aela's outfit is reset via property in the quest script attached to C03Rampage, when she changes back from werewolf form. You'd need to edit that property to point it to your desired outfit. I assume something similar is happening with Astrid, but I'm less familiar with the Brotherhood quests.