Yes, approach control or a tower sees the flight plan, distribution is from the center they belong to that handles their flight plans. FSS does not get a copy of an IFR flight plan if you file through ForeFlight, so are unable to amend a filed flight plan. If you were to specifically request an FSS specialist to relay a departure update to center, even though they did not have a copy of the flight plan, they would most likely do this on a land line for you. But otherwise, the specialist is going to respond they can't amend your IFR flight plan because they don't have access. There is no mechanism for them to call up your flight plan. I just called and verified this with an FSS specialist.