How do I switch my straight talk phone over to metro pcs when its locked?

https://www.straighttalk.com/wps/portal/home/h/legal/terms-and-conditions

VII. STRAIGHT TALK UNLOCKING POLICY

For Customers with Straight Talk Phones Capable of Being Unlocked : Straight Talk will provide an unlocking code to customers who request unlocking, provided they meet the following conditions:

(a) Customers must request phone unlocking. Straight Talk will unlock phones of current and former Straight Talk customers without charge. Non-former customers may request unlocking, but Straight Talk may charge a reasonable fee.

(b) The Straight Talk phone must have been activated on Straight Talk Service for no fewer than 12 months with Service Plans redeemed in no fewer than 12 months.

(c) Customers must possess a Straight Talk phone that is not reported stolen, lost, or associated with fraudulent activity.

(d) If a customer in good standing requesting phone unlocking is deployed as military personnel, upon provision of deployment papers, Straight Talk will honor that customer’s unlocking request.

Ask straight talk to release the carrier lock they are holding on it.

Contract or no contract doesn't enter into it. If the phone is locked to ST, then they are the only ones who can unlock it.

I brought my unlocked X to my Walmart and installed StraightTalk. I don't have any contract and can switch carriers as often as I like.

Any "locked" devices are locked before you buy it at Walmart and you must wait a year to remove their lock

https://www.straighttalk.com/wps/portal/home

StraightTalk doesn't have any contract so all you need to do is remove their sim and put in the other carriers.