What do I have to change in my code to immigrate from legacy stripe checkout to the new checkout?? I am confused with their wording. And most examples I find are old (2015-1016.
With the new SCA regulations coming in as you mentioned in your comment, you need to use Payment methods and Payment intents now.
Basically from taking a look at your sample code you will need to rewrite pretty much everything (if you haven't already)
Their current docs are here -> https://stripe.com/docs/payments/checkout
The reason for Payment Intents and Payment methods is due to SCA - https://stripe.com/docs/strong-customer-authentication
They have a migration guide too which can be found on the side bar.
However from their examples this is how you would create your new payment intent
$intent = \Stripe\PaymentIntent::create([
'amount' => 1099,
'currency' => 'gbp',
]);
Here is also their guide for migrating from the charges API - it has a tab for stripe.js V2 if you've been using it https://stripe.com/docs/payments/payment-intents/migration