Overview:
The Simple NPC Outfit Manager by confusedchim is probably still the best Outfit manager for Skyrim NPCs.

Simple NPC Outfit Manager allows you to have direct control over what your Skyrim NPCs are wearing. Create custom outfits and equip them to any Skyrim NPC. Powerful, functional, and lightweight.

It's only flaw is that once you apply an Outfit to an NPC, their default one is lost - you can never restore it, if for example you decide you don't like the outfit you gave them (or just simply want them to change back for now.)

This add-on extension fixes this issue.

How?
Once you give an Outfit to an npc, this add-on records its previous outfit and saves it. Then it provides a "Return to your original outfit." dialogue option at the end of Simple Outfit Manager's list.

Limits:
By default, the add-on can store data for up to 30 actively outfitted NPCs. If you somehow manage to reach this and give unique outfits to over 30 of them, the add-on will allocate a second stack, increasing this limit to 110 (with potential decrease in performance for each lookup. YMMV.).
If you are playing Sims with Skyrim and manage to cross even this limit, the add-on simply won't record data over 110 and will drop you a notification that until you return some of them to their default outfits and free up array space, your actions are not monitored.

Which edition?
Currently the add-on only targets the Immersive edition of the Simple Outfit Manager. But it doesn't matter which one you installed from it - this add-on overwrites the esp plugin and thus enforces the immersive variant, regardless of which one you downloaded from Simple Outfit Manager.
If you want to make this work for the utility version, after installation, follow the description on Simple Outfit Manager's modpage about how to edit the esp to turn on the utility functions, but make sure to do these changes to my mod's esp!
Unless masses come protesting at my door, I by myself probably won't make an official utility variant :)

Additional fixes:
The original mod only functions for NPC with relationship-rank greater-than-equlal-to 3 (ally), but for some followers (especially mod-added ones), this is not always true.
The add-on fixes this by allowing all functions for NPCs that are currently player teammates, thus the mod will function for 99.9% of all followers, vanilla or mod-added.


Requirements:

• The base mod of course: Simple NPC Outfit Manager
• And this add-on also requires SKSE!
  

Installation:
With your favorite mod-manager.

• Install the base mod (simple outfit manager).
• Install this add-on, and make sure to overwrite everything. The esp plugin for the mod should come from this add-on, not the base mod.
  

Manual installation not recommended, but if you know what you are doing and like to live dangerously: Extract the contents of the archive to Skyrim/Data. Overwrite everything when prompted.

Can be installed at any point, on any saved game.
But be aware: the addon defines "default outfit" as the one the NPC is wearing at the point of installation! Therefore if you already used Simple NPC Outfit Manager on an NPC, their "default" outfit will be the one you gave them back then, and their "true vanilla original" outfit is simply not recoverable by any means.

Uninstallation:
Nope, don't do it. If you absolutely must, there's safety instructions on the original modpage, but there's still no guarantee it won't break your save. Ye've been warned!

Compatibility:
Same applies as for the original mod, which is: should be compatible with everything.
Naturally if someone else makes another mod that edits Simple Outfit Manager, that that may be incompatible with this add-on (or not), but I know of no such mod.


Known Issues:
Only one, and it's not even really an issue, but well, for those who click around a lot randomly for whatever reason, this may be good to know:
For technical reasons and in order keep the number of edited scripts minimal, the mod doesn't actually record the npc when you say "Let's try Outfit X", but does it earlier, already when you enter the sub-menu: the npc data is recorded when you select "Let's talk Outfits". If you then select "Revert", the npc data is cleared of course, but if you tab out of this dialogue, the npc data stays in the array, even though you didn't outfit them.
Again, selecting revert will clear them and you can do that at any point, but.... thus it's technically possible to reach the 110 npc limit without even giving out one outfit.
Just don't do this - and problem solved.


And that's basically it for this one.
TL;DR: experimenting with outfits for NPCs has never been safer and reversible-er! :D


My usual pointless monologue at the bottom of the page:
Like most of my patch-type mods, this was made for personal use. I experiment a lot with clothes and how they look on followers, especially when I install new armor mods, but the the problem is that if the armor rating is below the npcs current Outfit, they simply won't equip it.
Simple Outfit Manager solved this issue beautifully, but not having the option to revert the npc to their original outfit was scary. I don't know if it would have ever been an issue, but simply the thought that if I apply an outfit - it's final... yeah, it was terrifying enough to make me do this add-on, and after getting permission from confusedchim, I decided to share :)