Actually, you CAN convert LPs to digital format, for CDs, mp3 conversion, whatever...
http://www.vinyl2cd.com/
These folks are just one of countless businesses that offer such services, or if you're feeling industrious, you can buy some gear and do it yourself.
So yes, downloading mp3s of music (even if you already own it on another format) through illegitimate file-sharing networks is still technically illegal. Personally I would find it acceptable on moral grounds, but..... illegal is illegal.
Mike