This article also covers 

Restrict plans from the customer portal

Disable drop-in script 

My users cannot change the subscription plan even after the "Allow customers to change plans" is enabled.

Summary

Your customers won’t be able to view Plans in their portal, in two cases,

1. If the option is Disabled in “Display in Customer portal” under “Hosted Pages & Customer Portal” at Plan level while creating a Plan
   
   
   
   

   1. To enable this, Go to Plans > Select the plan that you want to Display in Customer portal > Edit Plan 

   2.  Scroll down to Hosted Page & Customer Portal and select Display in Customer Portal.
       

2. If you have enabled “Allow customers to access checkout via API only” This option will disable the Hosted Page & Customer Portal section under Plans.
   
   
   
   

   1. To disable this Go to, Configure Chargebee > Checkout and Self serve portal > Disable “Allow customers to access checkout via API only and Publish.

   2. You will now be able to view the Hosted Page & Customer Portal section under Plans.
      
      
      
      

   3. Select Display in Customer Portal and Update Plan

Note:

Make sure you enable the Allow customers to access checkout via API only after you Create a plan and assign your API flow since the drop-in script function gets disabled instantly. 

Related articles and Documentation

Can I group plans and display them in the customer portal? 

How to restrict certain addons for specific plans?