Primary domain name is WWW.SITE1.COM
Parked name is WWW.SITE2.COM

External links from search engines+sites all reference in this format: WWW.SITE1.COM/FILE.HTML

I would like to swap the two domain names around so that WWW.SITE2.COM will become the primary domain (no problem).

What will happen when someone clicks on an external link to the old alias? For example, after the swap, if someone clicks on WWW.SITE1.COM/FILE.HTML will it automatically generate a 404 and simply load the ROOT homepage of WWW.SITE2.COM? Or will it load WWW.SITE2.COM/FILE.HTML which is what I want to happen?

Thanks