I always use from the spares and then replace spares with new strings, as Catherine has said above.
You didn’t mention how you keep your strings.
I keep each octave in its own plastic ziploc, all the nylons together in their ziploc, the guts in their ziploc, and the wires in their ziploc. So they are all double-bagged as well as being in their original little ziploc. (triple-bagged, really) Spares at home are in a protective canvas bag. Traveling spares are in a nylon bag.
I have two harps, so I have a double spares supply, but I use out of the traveling spares only….and then replace those with strings from the bag of home spares. I write the date that I got the string on masking tape and tape it to the ziploc. Some of mine (5th As and Bs) got to be 20 years old before they were used, and they were fine.
I never replace all strings at once, only what’s dreadful. I don’t have the time or money to do that.
If you’re getting rid of 5-yr-old strings, send them to me.
Here is my website to listen to my old strings.
http://www.WeddingHarpMusic.Yolasite.com