Can Shapechange stack abilities from multiple forms?

Question

The 9th level spell Shapechange allows the caster to change into the forms of other creatures. Furthermore, the spell allows the caster to use an action once changed to take on new forms.

During this spell's duration, you can use your action to assume a different form following the same restrictions and rules for the original form, with one exception: if your new form has more hit points than your current one, your hit points remain at their current value.

The spell lets the caster retain the benefits of features, not just from class or race but from "other source"s.

You retain the benefit of any features from your class, race, or other source and can use them, provided that your new form is physically capable of doing so. You can't use any special senses you have (for example, darkvision) unless your new form also has that sense. You can only speak if the creature can normally speak.

With these two passages of the spell combined, could the caster retain features of a form once they have shifted into another form?

For a concrete example, suppose a level 20 caster Shapechanged into a Pit Fiend possessing the Magic Resistance trait:

Magic Resistance. The pit fiend has advantage on saving throws against spells and other magical effects.

If the caster then used an action to change form into an Ancient Brass Dragon, which does not possess the Magic Resistance trait, would they retain the trait from their previous form as the spell lets them "retain the benefit of any features from [...] other source" and the new form is assumed "following the same restrictions and rules for the original form"?

Answer 1

This does not work, unless the DM wants it to work.

Sorry, I know the header is somewhat tautological, “it doesn’t work unless it does work”. A practical analysis should make it quite clear that it isn’t supposed to work as you describe, we will do that second. First I want to give a reading of the spell description that would prevent this stacking from working. Admittedly, this isn’t an airtight argument; there would be much deliberation in the court of rules lawyers about this, so I must lead with this: it is up to the DM.

Let’s read this very carefully:

following the same restrictions and rules for the original form

This means that the things we keep with the new shapechange form are the same things we kept with the first shapechange form. We must go through the same calculus we did initially when changing into a new form, and this original calculus did not include features from a form obtained through shapechange, so neither does this one.

But suppose the rules lawyer inside each of us would not find that argument compelling. If you care about balance at all (which in a tier 4 campaign, maybe you don’t, which is cool too), it should be obvious this doesn’t work. The question here outlines one application of this trick that translates to “no matter what form you choose you have magic resistance”. Once you know this trick, shapechange always gives you magic resistance, no matter what your final form is. But why stop there? Given one minute of prep time, we can line up 10 different creatures with an array of beneficial features and just keep all of them in our final form. I shouldn’t have to explain why this is broken.

To me, it seems quite clear that the function of the spell is “you get the benefits of your new form, plus the benefits of applicable features you had before you cast the spell”. Even if you don’t buy argument I make from the spell description, it should be clear that such arbitrary feature stacking is not the intended function here. So it doesn’t work, unless the DM wants it to work.