Changing CCK field value(s) by clicking URL - possible? Advisable?
It's feasible but not advisable. Drupal's standard for changing data via URLs:
1) Use _GET variables (or simple URL paths) to _access_data
2) Use _POST variables to change data

Along with using _POST, you would want to use Drupal's Form API so that tokens and fields are generated and validated.

Options you could do to change the data via URL (e.g. link) is to:
1) Add a token to URL to validate it's only used once (or other validation necessary)
2) Ensure user is logged in (and maybe ensure he is owner of that ticket, etc.)
Hope this helps.