I have found a census record that belongs to my great grandfather's family. I want to attach the record to my tree, but the last name is spelled wrong. If i attach, will it change the correct spelling in my family tree?
Nope and if anything changes, you can edit it and put it back the way you want it without detaching the source.
Short Answer: 'No'.
Attaching a "Sources" to an individual/person who exists in "Family Tree" of 'FamilySearch' DOES NOT "Change" the "Name' of the individual/person in "Family Tree" of 'FamilySearch'.
There are "Spelling", Variants; and, ERRORS, in MANY "Sources" that we attach.
Those, Variants; and, ERRORS, can occur for a number of reasons ...
[eg. Transcription; Phonetics (as, pronounced; &, subsequently, recorded); etc]
Go ahead an attach a "Source", if it relates to the individual/person, regardless of "Spelling", Variants; and, ERRORS, in the "Name".
I hope this helps.