Converting iTunes library to AAC?

I'm desperately in need to shrink my iTunes library from about 17 GB to something like 8-9 GB.
The easiest way I know to do this is to convert my library to AAC.
Now, I understand that AAC is a lossy format, and coming from another lossy format, there will be a quality difference, but I'm willing to convert to a high bitrate, I think 192 is the standard.

I know, in iTunes I can convert to AAC through selecting my entire library and selecting Advanced>Convert to AAC, however this doesn't delete the originals. How would I go about moving all of the mp3 files to the trash? Is it possible to *only* select a certain file format in iTunes?

Thanks for your help!