I want to change the strings on my strat to 8's so my little brother can use it to learn to play without bleeding all over it. I think i have nines on at the moment, so will changing down to 8's mean i have to set the guitar up again?
I didn't think it would, but i'm not an expert tbh.
Not likely, no.
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you would need to adjust the spring tension a bit, i think that the truss rod and the intonation will stay ok, but you never know.