Ability for auto.update of Cont. Vldt date when SO cerated with ref to Cont

Question

Hi Gurus!
When I create  a sales document from va01 with reference
to a contract and if a regularly scheduled customer calls
in to move up their service, I would like to have the contract dates( valid to)
automatically update per the schedule period .Example : if a 6 week schedule customer
calls in on the 4th week for the service, and he/she selects the lines from the contract,
he/she wants the system to automatically update the dates on the contract so that the next time
it apperas for service is 6 weeks from the date it chnaged from. Having to do it always manually is a
hectic task and consumes a lot of time and maybe sometimes maybe forgotten.
What I want to know is where , or which exit or enhancement point should I use to code in the requirement.
I tried looking at all the exist for va01 but could not locate where exactly it should go in.
Has anyone done something like this before and could help on it ? Would be really appreciated.
Thanks

Answer 1

Used  userexit_check_vbep to generate the message and adjust the schedule line contract dates.