How does Nick's statement "You're worth the whole bunch put together" show a change in Nick from the beginning of the novel?
I would say yes, in that in the beginning of the novel, Nick is very careful to state how he reserves judgment. His father reminds him that not everyone was brought up with the advantages that he enjoys, and the values that were instilled in him. He doesn't judge people, and tries not to hold their shortcomings against them.
This is a stark contrast to the way in which he has made a total judgment of Gatsby. Rather than passing a negative judgement or making a stereotype, he has judged Gatsby to be a good person, and tells him so. In so doing, as well, he has passed judgement on Tom, Daisy, Jordan, Myrtle, and George... stating that Gatsby was worth all of them put together.