If you want your songs in AAC, you should re-rip them from your original CDs. Don't just convert MP3 to AAC - it'll end up with something that sounds worse than either one.
Think of it this way. Here's a simulation of the data contained in a full, uncompressed CD:
So each letter represents a piece of data. But you don't need all the data to get a nearly equal sound. MP3 compression removes some of the data, like this:
A C EF H LMN PQR T WXYZ
So, all those gaps represent areas that were removed by the MP3 compression. You save space (in actuality, you save a lot more space than I depicted).
AAC removes different sections of data:
CDEFG JKLM OP RST W Y
AAC removes the same amount of data as MP3. However, AAC is smarter about it, and it removes data in a way that produces better sound at the same amount of compression. In both the MP3 and the AAC file, I removed ten letters.
So what happens when you convert an MP3 to an AAC file? Well, you take the MP3 data (which has already had parts removed) and remove any parts that AAC would also remove, leaving you with this:
Whoa! I just went from ten letters gone to sixteen gone! MP3 and AAC compression work in a similar way in that they remove certain frequencies from the uncompressed audio. If you convert to MP3 and then change the MP3 to an AAC, that means that any frequencies removed by either one will not be in the final music file.
In short, re-import your CDs in AAC format if you want AAC.