If I buy an iPhone from Straight talk that has the Verizon sim card, could I switch back to Consumer Cellular later if I wanted to?

If I buy an iPhone from Straight talk that has the Verizon sim card, could I switch back to Consumer Cellular later if I wanted to?

Only if the iPhone has been unlocked to all networks.

Many are supplied in that state, others are supplied locked to a single network. Even those can later be unlocked once the owner has completed a minimum subscription period with the original network. After that, for a fee, the owner can ask their network provider to obtain an unlocking code from Apple. There are some other means of unlocking iPhones but they're not guaranteed to be permanent unless Apple supplied the unlocking code.

Apple always charges a fee to supply unlocking codes, but some network providers waive them. In some legal jurisdictions suppliers are obliged to waive the fee once a minimum contract period has expired. But they just add it onto the monthly payments throughout the contract instead, so it's never truly free.