In Hebrew 5:9, what changed about the Son's perfection?

In the book of Hebrews, we read: >8 Son though he was, he learned obedience from what he suffered 9 and, once made perfect, he became the source of eternal salvation for all who obey him 10 and was designated by God to be high priest in the order of Melchizedek. Hebrews 5:8-10 (NIV) Here, the writer to the Hebrews describes God the Son as being "made perfect". The simple reading of this would imply that he was *imperfect* before this event, but that would contradict my understanding of the rest of scripture. How would a reformed protestant understand the idea of God the Son being "made perfect"?