I just got an email back from the student loans government website. If you messed up the loan amount for a PLUS loan, you have two options, both of which involve calling the financial aid office at your school
1.) Make the second disbursement of the loan come in the fall semester instead of the spring.
2.) Change the entire loan amount so you have adequate funds for both semesters.
I did the latter, doubling the loan amount so I actually get $3,250 a semester. Literally took a 2 minute phone call, I was amazed at how quickly you can do this