We are unable to install a different theme on your active subscription than the theme you subscribed to due to how our licensing and download policies work.
You are able to cancel your current plan and re-subscribe to the new theme plan within 30 days and we will be happy to refund the difference.
You CAN buy any other theme from themeforest and install it on your current plan yourself. Please note that we can only offer support for themes that include the Hosted option.
Should you be interested in cancelling your subscription and resubscribing with a different theme, we recommend contacting support to discuss your needs. They can point you in the right direction regarding migration of your existing site, and/or any potential impact to content, widgets and page layouts when changing to a new theme.