Decompressing Mp3 Format

would converting mp3 to flac restore the original full lossless quality of the recordings?
i used to hear more depth & clarity, presence with the flac format, but my hearing has been a bit degraded over time in my senior years, so i have a difficult time trying to distinguish the different formats.
i have a collection of jazz & classical symphonies's that i would like to hear in their full glory once again over my 901's

 

No.
MP3 is a lossy compression format. Lossy compressors (or encoders) use algorithms, and they also remove sounds that are masked by other sounds or are considered inaudible to the human ear.
When converting from a lossy format to a lossless or uncompressed format, the best you can expect is the same sound quality. But, that's not a given.