In Great Expectations, why have Trabb and Pumblechook changed their attitude towards Pip?

The simple answer is money.  Before Pip's acquisition of
wealth, Trabb and Pumblechook didn't give Pip a second thought, and even treated him
with a bit of derision.  When Pip comes into his money, Trabb and Pumblechook decide
that he might be worth their time.  Pip goes into Trabb to buy some new clothes, and
remarks that Trabb "did not think it was worth his while to come out to me."  At this
point, Trabb did not know that Pip had money; he was eating his breakfast and didn't
even stand up to help Pip.  However, when Pip tells him of his wealth, Trabb immediately
sets down his breakfast and Pip remarks, " a change passed over Mr. Trabb."  Trabb then
goes out of his way to behave respectfully.  The only difference here was the fact that
Pip had money.

It is the same with Pumblechook.  Before
Pip's money, Uncle Pumblechook was a bully to Pip; he teased him, scorned him, called
him names, and beat up on him a little.  After Pip received his money, Pumblechook
changed his tune.  All of a sudden Pip was "my dear boy," and "my dear young friend." 
He becomes submissive, asking Pip's permission for everything, and goes out of his way
to feed him and be polite and solicitous.  Again, the difference here was Pip's
money.

It just goes to show how much power money can have;
it also shows a rather unflattering side of human nature, one that indicates we will do
quite a bit for money.  I hope that those thoughts help; good
luck!