If you are using FM11 or higher, in layout mode pick your field and select Format -> Set Script Triggers, and set a script to run onFieldModify. When you change the service type, you can trigger a script to do a bunch of other things.
You will need to set up some tables where you can link a record for each service to a table listing the tasks for that service. This in turn needs to be linked to a table for your customers and a table where you select a service for that customer and then record when each service specified task is completed.
So what tables and relationships do you have in place at this time?