Incredible is a CDMA phone, 3G phones with sim cards are GSM - those are radio standards as different as AM and FM.
I know at one time, Straight Talk supported both, but I don't know if they still do. If Straight Talk is GSM only, forget it.
Getting CDMA phones to work on other networks requires changing some identification numbers that don't change with a rom or operating system update - think of these numbers like the VIN number on a car.
Sometimes that carrier transfer may require changing values that are said to be illegal to do, while others say a complex identity transfer process may be legal.
AndroidForums.com staff can't advise on anything that openly violates the law or your carrier's terms of service, nor can we act as your legal counsel.
According to this vendor, they can assist in navigating those waters legally -
Flash CDMA Cell Phone : Flash Cell Phone to Cricket, Alltel, Verizon Sprint
You may wish to chat with them. We are not affiliated with them in any way, and this is not an endorsement, merely a possible suggestion.
There is a change in copyright protection exemptions that went into effect on January 26, 2013, and some reports indicate that some dealers of those services are being targeted by the legal arms of various carriers where violations are occurring.
If you go for another phone, with changes in the laws, beware of people selling you phones telling you that unlocking is easy because Mr. Google says it is, check here, someone will be happy to help!