Your suspicion is correct -- what was once loss cannot be recovered withou re-ripping your collection. The file size is bigger but there isn't any additional information content there.
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There have been several threads about this here and there. Turns out that even if your file was originally ripped in Apple Lossless, you are still offered the choice to "Convert to Lossless".
In which case ignore.
If the file was originally ripped as AIFF or WAV - uncompressed lossless formats - the command is valid. It will create a new lossless file about half the size.
If the file was originally ripped using a lossy compression scheme - eg MP3, AAC - some percentage of the data was lost during the ripping process and as you correctly suspect, cannot be recovered.
I would guess that like a jpg, if you were to save a lossy file in Apple Lossless it will preserve that degree of resolution without any further degradation. The problem with a lossy format is that a little more gets lost everytime it is saved.
That is why a percentage of people on these boards prefer to rip to WAV or AIFF despite the added file size and in the case of WAV, inconvenience. They want to be as certain as possible (this is technology after all and change is inevitable) that they will not have to go through the ripping process again no matter what cool new format emerges in the years ahead. (Personally I think its overkill, but it's harmless overkill and they may well have the last laugh on me.)
BTW if you are now considering re-ripping your collection to Apple Lossless, be sure to do so with Error Correction On.
And yes, there are even more posts to be found confirming that Apple Lossless is indeed Lossless and that in fact absolutely nothing is lost in saving to Lossless...
Though there are always those out there who insist that they can detect more air around the left tom tom head with AIFF then Apple Lossless etc. If you are one of those golden eared listeners, ignore the algorithim, the sum checks and all the rational techniques available to confirm the engineers assertions.
Because for you it is not lossless. Saving to WAV or AIFF are the only acceptable formats. After which you can push Convert to Apple Lossless LOL