Not really. Once a file is low bitrate, you can’t magically make it sound better without a lot of work (and sometimes there’s no chance at all).
Also, there’s two types of DJ in this world. those that have downloaded “free” music at some point, and God Damned Liars.
Best DJ quote ever ?
I can honestly say that I have never downloaded “free” music.
But, I am definitely one of the 2 types of DJ mentioned above. 
Don’t ever “upgrade” from mp3 to WAV/AIFF/FLAC/WHATEVER lossless format. As an MP3, it’s already lost some of the audio quality. You can’t get that back, so it would be a waste of time to attempt it.
Yes and no. Vinyl is analog…but that does not imply “infinite” resolution or bit depth. This is picking at nits…but even so…analog recording mediums have well known limitations (and a few benefits).
I agree that FLAC is a good choice for archiving music. I find that the “default” settings yield a file size that is 50%-60% the size of the corresponding wav file.
If you mean re-ripping from the original source and storing, then “yes” it is worth doing. I would rip to FLAC for storage, and convert from there to whatever you want to use while playing a gig. Over the 15-ish years I have been using a computer to play music, I have ripped my CD collection four times…each time to improve the quality while staying within the space constraints of the day. At this point, I have everything ripped as FLAC…so in theory, I will not need to go back to the originals every again.
Yes and no.
There is a common mis-conception that larger sound systems are “higher fidelity.” They are not…but they are typically louder than most home-listening systems. The highest fidelity, and the best chance to “spot” a lower quality recording, will come in a small(er), acoustically treated room, with quality headphones or “flat” monitor speakers. Adding the background noise of a car going down the highway, or a club with 500 people who are talking and carrying on…is NOT the best environment to use to pick out low(er) quality source materials.
However, the Fletcher-Munson effects does bring out some details in the music at higher SPL levels than is present at lower SPL levels. That can make lower resolution recordings stand out at higher SPL levels.
The generic advice remains, use mp3@320 or the lossless format of your choice for all music.
Do not convert. Re-rip from the original source material.
6 -v- half a dozen.
Not in practical terms. mp3@320 is about 2.5MB per minute. Wav is about 10MB per minute. FLAC is 5-6MB per minute.
I can buy a 1TB external drive for less than $100. Hard drive space is effectively “free” compared to the cost of the music.
I still have my collection of bootleg cassette tapes from the 80’s.
There are some narrow regions where the conversion makes sense. Most audio editors will convert any file to wav (or equivalent) in order to edit the song. If you are planning to incorporate a bit of audio that you only have as mp3 into a music editor…doing the conversion ONE TIME from mp3 to wav makes sense. This will NOT recover the lost information, but it will prevent additional losses from multiple conversions from mp3 to wav and back.