Hello all !
I would like to know if there is any software able to convert my aac cbr tracks to aac vbr ?
I know a software which is able to do that but only for MP3 files, it's mp3packer, and it does it losslessly !

If someone could help my that would be great!

The nature of AAC does not allow this without re-encoding. In MP3 this is only possible because 1 frame of audio data is allowed to be spread over multiple frames of MP3 data. In AAC 1 frame of audio data is always 1 frame of AAC data.

Does that mean AAC has no bit reservoir?

You can implement an AAC encoder without, yes. But if you want to have true CBR, you will need it. In AAC you will just have different framesizes that are constant bitrate when combined, while in MP3 the framesizes remain constant, but each frame will hold a fractional part of the audio data for 1 frame.