if i have upgraded sits 400 ppl & if i upgrade my stadium, do i loose that upgrade ?

[Accyrover]

The least expensive and shortest time to complete stadium upgrades is by; completing the seating section all in one go, then do the stadium, you save around 10% cash and one - 2 days on the stadium; as opposed to doing the whole upgrade in one go.
It is of course a wast of time and money unless you have the fans to fill it, even then at L1&2 it will not pay for itself for more than a season.
Also the seat upgrade to an existing stadium has a lessor capacity than the full stadium upgrade.
To maximise either scenario you need to upgrade the car park, I'd suggest doing that first.

Having said that, players needs come before fans in this game.