i cant 100% say you should or you shouldnt change all the strings. i guess it depeneds on you i guess. i always change all the strings but im really OCD about that kind of thing. i also dont buy strings in bulk and i hate finding sets with a string missing or throwing away 5 brand new strings.

if you havent already tried you might consider buying a few sets in bulk just to try them out. also if your strings are rusty you should change them all cause that rust can sap out your tone.

one last note if your buying strings in bulk figure out which ones you break the most and buy a few extra of those. so you might buy 3Low E 3A 3D 5G 4B 6 high E. (i love bending the G and E so i break those the most.)