can i use new oem disc and old serial

I have googled it till im sick of searching….i am just curious if i bought a new oem home or pro disc …..and i did a reformat and reinstall on a friend computer could i use my new disc but his old serial ….im not sure if he’s got home or pro…gotta check …and if it works is it legal? ….also how do repair shops do it,do they have a volume licence or are they doin like what im talkin about…because the ones around here charge like 120 bucks so you know your not gettin a new oem when they reinstall…..please help……thanks