ATF states:

"If the licensee or the buyer/transferee discovers that an ATF Form 4473 is incomplete or improperly completed after the firearm(s) has been transferred, the licensee should photocopy the inaccurate form and make any necessary additions or revisions to the photocopy. The buyer/transferee should only make changes to Sections A and C. The licensee should only make changes to Sections B and D. Whoever made the changes should initial and date the changes. The corrected photocopy should be attached to the original ATF Form 4473 and retained as part of the licensee’s permanent records."

When making a correction on a 4473, does the whole form have to be copied (including notices,instructions and definitions)? Or can just the affected page be copied.

I think it's just the page(s) with the errors. And then attach it to the original. But wait for someone more experienced to respond.

Make a copy of the entire 4473.  Many IOIs wont care but I've seen citations for it.

Just copy the pages with changes. Pages that do not have written information do not matter.
It is a notation that there was a change. Nothing more. You have met the expectation the rule was looking for.

Only the copy the page with the errors. 

Please make a copy of the entire form.  I am a firearms attorney and represent 100s of FFLs across the country for a living.  I have seen FFLs cited with violations, more than once, for failing to copy the entire form!  

The regulations (in 27 CFR 478.21) say to "photocopy the inaccurate form."  This has repeatedly been interpreted to be the entire form, not just the section with errors.  Note, it doesn't say to "photocopy only the section of the form with errors."  The ATF will use this complete photocopied form if criminal charges are ever brought against someone as the most up to date/accurate version of the 4473.

We can debate whether this should be the case, but the fact remains that this is how ATF has interpreted that section of the law.  You can argue with the ATF how you think it should only be the particular section that needs to be photocopied while they write you up for a violation.

If only a photocopied section is included, how is the ATF to know, and how will you prove, that there wasn't another page that was photocopied with corrected errors also that was later lost.  Only with the complete form photocopied can you show that there weren't corrected errors in other sections too.

I'm not saying that this makes sense or even how it "should be" - I'm just telling you how it is.  Even if you think I'm wrong, there's no harm in photocopying the whole thing.

NOTE: I just recently found this section of the forum and am here to help out however I can.  Although I am a firearms attorney, my answers on this forum do not constitute an attorney/client relationship and this is not legal advice.