I've seen a couple of answers to similar questions (e.g. How do I set the value of external_id to be the same as a custom field? and Membership fee based on custom field).
What I want to do is change the content of a field from www.fubar.com/id= to www.fubar.com/id=[CONTENT OF FIELD] I thought this was doable with CiviRules, but it appears there is no option to change the contents of a field based on the value of another field change (I can do other things based on a field value change).
Is this an area where I need to look into custom development? Or am I missing the bleeding obvious?