You can do what you're wanting with the CBSubs Fields plugin, it's specifically designed to change field values based off a users subscription; this integration is included with your CBSubs package. You can also use CBSubs SQL Actions as you've been doing or use CBSubs URL to execute a script file; you've many options available to you. The simplest of them is CBSubs Fields.

kevincam wrote:

Where is the CBSubs URL option you mentioned? I'd like to execute a script upon successful plan activation. I need to change expiration date as my plans all expire on the same date (8/31) but are annual. So I plan to write a PHP script to analyze the date and set the expiration date accordingly to 8/31.

It's included in your CBSubs Package; please refer to your documentation for installation instructions.