You can start with a plan which best suits your current needs and upgrade anytime if you need a higher plan or downgrade if you don’t need the features of the higher plan anymore.
The plan can be changed from the workspace dashboard’s Manage / Workspace settings menu / Plans and billing tab.
When you'd like to downgrade, you may get the following message:
Please have a look on the message on the top of the screen, it will look something like this:
In the example above, you can see that the Jira integration is enabled on the workspace and there is at least one map with Trello integration enabled, which prevents changing to the Basic plan.
You can disable the Jira integration from the Manage / Integration settings menu, by clicking on Disconnect JIRA button:
To remove other integrations tied to the story maps, please open Manage / Manage storymaps menu and you'll see which map has any of the integrations:
You can ignore the Jira integrations here, if you've disconnected from Jira at the workspace level.
You can remove the integrations from the Options menu:
After removing the integrations you should be able to choose the Basic plan: