The SIM card in most CDMA phones is for data, not a phone connection.
Flashing changes the firmware, not the carrier.
If Cricket uses a GSM carrier, you can't use a CDMA phone on their network. If they use a CDMA carrier, it's up to the carrier whether they'll accept a particular phone. (For instance, Verizon won't accept a phone you bought as a no-contract Verizon phone.)