Updating properties dynamically from velocity

Hi all, Not sure if this can be done; but I've come across the need to update object properties directly from velocity. For example, object A is in a particular document 4 times. Each has 25 sets of 2 string properties, (field1_before; field1_after) When the user updates the _after field, I need to do certain operations - when done, I then need to update the _before field so that it only happens once. I guess, long and short of it is: anyone managed to update a property from velocity directly (without redirecting the url each time)? Or have any idea's how it could be done?