The order likely didn't change and ARMS was always meant to be first. They use characters as "bases" when making the DLC mostly for an effect the characters share. For instance, Banjo used Duck Hunt as they both have a character attached to them. Byleth uses Simon for the whip. This visual glitch is on Bayo and its for her Smash attacks. This is likely being used for Bayonetta and it fits both Min Min and Twintelle really well. Also, it would be hard for them to change the order as the out break was only a month and a half ago. More likely its just delaying the release.
You say it likely didn't change, but the reality is that you mean that you don't think it did, not that theres any reason to believe it didnt.
You could argue that it was default.
However, without question, we can say Corona delayed the game. We know this as a stated fact from Nintendo and Sakuria, both.
This means that we know work was delayed.
What we don't know is 'what' work was delayed.
If it had been a planned third party character as the first in the pass, it would have been almost impossible for the character to have come out anytime soon.
Sakurai has outright stated he meets with developers in person, in part due to cultural (or even language) barriers.
This became impossible with Corona, months earlier in Japan than elsewhere.
Now, would it make more sense for NIntendo to wait many more months for the next character reveal, just to stick to an arbitrary order, or would it make more sense for Nintendo to switch up the scene and the order?
So in short- we know Corona affected the game, we know that a third party character that could have been planned first would have caused an even bigger delay than a first party. NIntendo's options were to go with an in house character, or wait even longer for the new character.
This shows us high possibility that the characters were switched around.