Will the song have worse sound quality?

no

Lossless is always lossless.

Encoding to a losless format will not lose any quality however many times.

Although, encoding to lossless from a lossy file will result in the same quality as the lossy file - audio quality is not regained.

Assuming there are no issues or errors in either decoding/encoding process, lossless to lossless, should always be... lossless..

If a file was encoded using FLAC level 5 and then re-encoded to FLAC level 8, no problem?
Or from APE to FLAC, no problem too? People seams to like more the FLAC format then the APE.

Please read posts #2 to #4.

If you still have questions, please re-read posts #2 to #4.

I read those post, it was just some kind of insecurity of my self. Thanks for replying.

To rule out the possibility that a misconfigured player (frontend) might have altered the data, just run a couple of first albums through a comparator.