I need to contact Trenitalia together help with correcting seat assignments traveling from Milan to Venice. Any ideas?
Trenitalia can't correct seat assignments.
But once you are on the train you can of course move to a different seat (if free), swap seats with other passengers etc...
Can you find your reservation online under Manage your tickets or on the app? I seem to remember being able to changes reserved seats for already purchased tickets but it's been a while.
And WengenK is right - changing seats is common. I've seen conductors come to check tickets for a "new passenger" only to have the current occupant point out the new passenger in their original seat. As long as the passengers agree no one cares.
Good luck,
=Tod