Global Password Reset on WP/BP site?

I have migrated a large network (800+ members) from Ning to a new WP/BP site, and it's about ready to go live.

We now need to decide the best way to let users know that their account has moved from Ning to WordPress, and to allow them to reset their passwords to a new password of their choosing.

I wondered if anyone out there may be able to suggest the most elegant way to handle this global password reset? Obviously, I could email all users via MailChimp to let them know to go to the WP login page and hit "Forgot password", then get WP to email them a new password which they then need to change... but that's just messy.

Are there any plugins that will handle this more elegantly? Ideally, I'd like to inform users of the change by email and then link them to a page on WP/BP that would allow them to quickly select a new password and login to their profile to make updates?

Any thoughts on the best way to get users up and running quickly with a new password on WP/BP gratefully received.