Can i take back my Verizon Gusto, and buy a new smartphone within 30 days?

I went to Verizon, i already had a HTC droid Eris, but it was reported lost, (long story because of my mom) so when i got a new Verizon account, They gave me the Samsung gusto. When i went home i talked to my parents, got my phone off the lost list, so i activated it with the account. Now that i have been thinking if i can come up with 300 dollar, can i go into Verizon and switch my Gusto phone for the Fascinate or Incredible. Will this work. I got a plan Dec. 29th so i still have 20 something days