Preventative of what? In case they were compromised and don't know it? I have zero reason to suspect my own password file has been compromised, so there's no reason on my side to change it.
"We learned about an old set of Dropbox user credentials (email addresses plus hashed and salted passwords) that we believe were obtained in 2012. Our analysis suggests that the credentials relate to an incident we disclosed around that time."
they screwed up. yes, it was 4 years ago but they did. they should acknowledge it in the most simple manner.
Or they have an issue they don't want to talk about and this is one way to get it resolved...
That's normally how you'd do a hashing upgrade
* decide an upgrade window
* each time a user logs in check their hash version, upgrade if necessary
* the upgrade period expires
* if you can, force log out all non-upgraded users - hoping they'll now log in and get auto-upgraded
* email all non-upgraded users and ask them to update their password
Hopefully that set of users is now very small.