Hi, have you considered getting iTunes match? if your library a reasonably tagged correctly then iTunes match can simply work its magic and back up your songs and keep all meta data. So you simply delete the song on your computer after its been matched, at this point you can listen to it in the cloud or simply re-download it and it replaces it with a 256kbps quality file and retains the date added etc.
This is what I done since I also organize my library from date added. Anther way, if you are wanting a higher quality file to replace it, then right click on the song, view in finder and then delete the songs in the album file, then replace with the higher quality file. Itunes then gives you a "file cannot be found" and you chose to locate the new file and again keeps your date added while upgrading the sound quality. This can be quite time consuming, but if you're OCD about your collection you will end up putting the time in, lol.
I hope that made sense. 🙂