In the late 60's early 70's my family purchased a lot of classical music on Reel To Reel tapes. About five years ago I was involved in digitizing them. Located a reel to reel tape machine from the basement of a local radio station. Got fresh leader tape and a splicer. The quality of the most mundane tapes was excellent, particularly DECCA. London ffrr were about to delaminate. Was able to recover the music from them (Gilbert & Sullivan) by BAKING the tapes.

I agree that magnetic storage (on good quality media) is typically excellent for longevity. CDs and DVDs have yet to prove themselves. If the tapes are precious to you don't get rid of them, store 'em in a cool dry place with good drainage in case of an emergency.

The first stage of conversion is to capture the music into WAVE files. This is highest in memory intensity and in storage capacity. BUT, good hard drives are cheap these days, and you probably should keep the WAVE files while converting them to the mp3s or whatever format you want to listen to them in.

Good luck with your project.