To change your Celtx plan, log into your studio account at celtx.com and select 'Management' then 'Plan' from the left hand side of your screen.
In the Plan section, you can choose to upgrade or downgrade your level of service.
Upgrades are available in the "Improve Your Creative Experience" section. You can upgrade to a plan with more production tools, add on episodic tools, or add extra projects to your plan.
Upgrading your service will calculate a pro-rated purchase difference.