Upgrade any time through the billing page. The change takes effect immediately and you're prorated for the difference.
Pro is worth it if you need:
Coupon codes for promotional campaigns
Create a custom lab panel for your clients
Offer on-site phlebotomy (with a certified phlebotomist on staff)
Offer mobile phlebotomy (to your client's home)
Priority support including phone support
You can always upgrade or downgrade at any time.