Hi I just did an engine swap due to overheating super low mileage only 120k 🙁 but after swap I noticed check engine and vsc and code p0778 it goes up only to 4th gear I think or 3rd and when it’s coming to a stop it downshifts super hard really weird it is a 2006 ls430 anywhere I should start?

If shifting correctly before engine swap, then check transmission side wiring harness and connections.

Do you know which connections they are ? I already took the car home from mechanics.

Sounds like it needs to go back to the mechanic for them to check that they did everything correctly.