MP3 to another file format conversion?

[brillhart]

I have a huge collection of music.
Some of the files have very low quality sound.
I know that if we convert an original CD track to an mp3 file or any other format (except WAV and Monkey's audio), quality of the decreases.
by I don't know about the reverse, conversion of mp3 files to wav or higher quality audio formats.
is there any enhance in quality.
please tell me.
also help me know about the quality change when an mp3 file is converted to a higher bitrate.
is there any change in quality occur.
Answer this question EXCLUDING the thought of file size, include only quality.
if there is any quality enhancement, then what would be the best configuration for audio conversion.
i use NCH, the great audio converter.
Please answer my questions thoroughly.
thanks