It can, but not retrospectively.

"By continuing to use Bandcamp, you agree..."

What's an artist who depends on the platform to do?

Move their music off it. It happens a lot. If a band's earlier Bandcamp album gets picked up by a label, they will insist on exclusive distribution, so the band will remove it from BC.

To where? Because as a buyer of music I like BC's deal and would love to make sure alternatives have similar arrangements?

That would be the worst move Songtradr could make here. The ability to distribute your music while maintaining the rights to it is one of the main reasons - hell, if not the main reason - artists and labels (and fans like myself) love Bandcamp. Making such a change would, to my mind, royally piss off the entire community and, in particular, the artists that make the platform as successful as it has been. I can only imagine the negative press that such a move would generate.

Leave.

If Bandcamp were to try such a move they would destroy the value of their business overnight. Retroactively changing the agreement with your customers about their IP is the best way to permanently destroy trust.

It would be a more dramatic overnight loss of customers than Unity. It's entirely laughable, and given that laws restrict the licensing and royalties of music, it might not even be legal.

Labels can only write up contracts giving them rights because they pay for studio time (given them a financial buy-in to the creation of the music) and pay advances against royalties. Not so for indie distributors. I don't know if there are specific applicable laws, but it's absurd enough that everyone would immediately drop Bandcamp.

Music royalty rates, for physical records, radio and internet streaming, are also determined by a panel of judges: the United States Copyright Royalty Board.

"Pray we do not alter it further..."

I guess that only works for Darth Vader.

Not saying they won't try, but there are enough label-represented bands on Bandcamp that they would obliterate their back catalog (along with any network effects they're enjoying) overnight.

Technically Bandcamp is a very simple payment and file host.

A small team can quickly copy how it works and get all the artists to move platform if they were to shoot themself in the head. Right now this doesn't happen because Bandcamp functions and exists and has a reputation for musicians.

Lose that, all the artists start looking for a new home and someone will make an alternative quickly.

See: Unity

If it did, I'd probably stop using Bandcamp.