Yes, you can change user permissions anytime you want if you are an account admin. It's pretty much the same process with removing a user's access.
Step 1: Click on the "Settings" tab in the main menu.
Step 2: Click "Users" under the Settings menu.
A list of users will appear.
Step 3: Go down the list and when you reach the user that you want to edit, click "Manage User Permissions".
Step 4: Modify the status and/or permissions.
You can change the "Access Level" on the right from "Admin" to "User" or "None" or delete the user. "Admin" user level has the access to all of your ad accounts and companies. If you select the "User" option as the access level, then you can give access for only selected Company/Brands and Ad Accounts. Choosing "None" would effectively revoke the user's entire account access and prohibit her from logging onto the account.
Once you are done, do not forget to click blue "Save Permissions" button.