Basically, shapeshifters in various fictional universes tend to have a preferred form, sometimes it's also their "basic form". We have MLP changelings, for instance, who clearly have their basic form.
Another example would be World of Warcraft's druids - either elves or other race - they switch into their shapeshifting forms but ultimately, their basic form is their humanoid one. You might also know the trope that shapeshifters turn back to their original form upon death (like in Rick and Morty's episode "Total Rickall").
I remember a cartoon where a certain species of shapeshifters forgot their original form...
That being said, the whole idea of shapeshifting revolves around spending time in a form different than your original one. Now shapeshifting doesn't really occur in real life, so what shapeshifting is really like, in fictional universes, that is?
It varies between fantasy worlds but it might be something akin to wearing a costume or a mask irl, or something like channeling energy to maintain some kind of illusion... something like running? Or maybe something like controlled "manual" breathing? Or maybe it's not a big deal for a shapeshifting species but they are still aware they are not their "True self"?
Whatever the explanation is, I think it'd be safe to assume that shapeshifting requires some sort of effort or attention... or other awareness. A shapeshifter's form might be a perfect disguise but it's still a disguise.
Imagine being forced to wear a fursuit 24/7... or maybe even just a Batman costume all the time. Or a clown make-up?
Recently, Ocellus cried when she was turned into Chrysalis... she couldn't stand her "new-old-enhanced" form because she preferred her normal, colorful one
Now for the poor Berzie. Can he change into his old form? Yes, and no. Yes, because Dopple could, so can Berzie. But Berzie doesn't remember his exact features. You might say it's a ridiculous statement but now... can you draw a self-portrait without looking in the mirror? Or how perfectly can you recall your exact shape/size/weight...? Or do you know the color code for your skin, hair, eyes... Well, changelings seem to easily copy nearby creatures but... Berzie doesn't have a replica of his old form because it was never needed.
However it's safe to assume he could switch into an average black changeling.
Yet, shapeshifting requires effort and whatever shapeshifting is like, it most likely also requires some effort and might create a psychological discomfort or other dissonance.
As for the effort, Berzie is shown to be quite lazy. You can compare it to girls who choose not to wear any make-up, maybe? Or to guys who don't shave too often
And don't make me start on the psychological side of the problem
Speaking of laziness, there was a comic where Berzie got new teeth. Some couldn't understand how getting teeth done by a dentist is less lazy than shapeshifting. The answer seemed obvious to me - shapeshifting requires personal effort and possibly requires to maintain the illusion. With the dentures, Berzie can stay in his 100% basic form and still have fangs.
Well, some people seemingly got the idea behind this. Some others, however, didn't look at the big picture. Or didn't take "reality ensues" trope into consideration.
Useful reading on tv tropes:
Shapeshifter Default Form - touches the concepts of True Form and Preferred Form
This Was His True Form
