How to Sell Discounted Subscription Package for 1st 3 months & Then Roll Customers onto Regular Monthly Subscription Plan at End of 3 Months?

Hey everyone. I’m developing a simple app that does 2 things:

  1. Charges their card now for a discounted 3 month plan for the first 3 months (giving away 1 or 2 months free with the pre-orders) + adds them to a wait list for when we launch the app
  2. Automatically rolls their plan into the regular pay every month plan after their 3 month intro package ends

To do this I created 2 different products in Stripe - one is a one time charge for the recurring plan & the other is a recurring monthly charge.

How do I add the subscription to the monthly recurring plan when they check out with the purchase of the 3 month intro? It’s not a trial period really so I’m not sure how or where to delay the date of the recurring monthly charge.

Also, if the app 2.0 development (the one they’re subscribing to) ends up getting delayed & I have to push back all the plan start dates, is there a way to change all plans in Stripe?
Thank you!