Actually, converting MP3s to AAC will likely DECREASE the quality, when I did it it was very noticeable and I am not an audiophile by any stretch of the imagination.
The reason the quality will decrease is because both formats remove information from the audio but the information is thought to be very hard for humans to hear, so you get smaller files but without much of a drop in quality as you couldn't hear what was removed even if it was there. However, both formats remove different data, so when you have an MP3 it's missing data from the original recording and it isn't that big a deal but when you convert that to AAC you will loose even more of the data and it's when that happens that the removed data becomes very obvious. One way to get round it may be to convert it to a high bit rate AAC but then that would totally defeat the purpose of trying to save space.
I hope I answered your question 🙂