If you'd like to offer your first batch of paid members an introductory (discounted) price, but plan to raise it later for future members, you have 2 options:
Set the price for your paid product to the introductory (discounted) price now, and when you're ready, you can increase the price by visiting Settings > Monetization in Heartbeat and clicking the 3 dots to the right of the paid product to edit it. Early members will continue paying the discounted price & new members will pay the higher price.
Create a coupon code in Stripe for your introductory members. You can limit this code to "only 20 uses" to create some urgency for members to join now vs. later. They can use this code during checkout to purchase access to your community. (See How to create promo codes for Heartbeat payments)