Is it an engagement ring you have? If so, I would definately say don't CHANGE it because it would never mean the same, although it was given to you by the same person.
If he really needs to buy you a bigger diamond then why not have a diamond necklace or even another ring for another finger but I would definately not change your engagement ring.
My engagement ring is small - I can't even remember the carat size but it was all my husband could afford at the time, and although I wish the diamond was bigger, I know that spend all the money he had on this ring.
Now that he can afford bigger diamonds I'm just going to have a hugh eternity ring for our 10th wedding anniversay (especially as he has made me wait 10 years for it!!!!!!).
Definately, go with your heart if you like the one you have now - keep it - get him to spend money on you in a different way.