WordPress thumbnail settings change

I have a very large WordPress image bank (over 600 images, all in use…!) and a new development will mean that i would like to change some of the thumbnail settings.

I have at the moment

set_post_thumbnail_size( 75, 75, true );

In the functions, which of course means that the thumbnail is cropped to fit. I would like to change this so that the thumbnail is scaled, rather than cropped

set_post_thumbnail_size( 75, 75);

But will this change the current images i have? If not, is there a way of doing this?

Thanks!