Implications for switching licenses mid-stream

[Jon Lesser]

I am working on an open-source web application that has been licensed,  
since it's inception, under license X. Now we're looking at switching  
to license Y.

Many people have already downloaded our software with license X. Can  
those people continue to use what they've downloaded under the terms  
of X even after we change the license to Y?

The X and Y cases I'm particularly interested in are X=BSD, Y=GPLv3  
and X=GPLv2, Y=GPLv3.

Any insights would be greatly appreciated.

-Jon