When upgrading should i dezactivate all plugins?
I do, some people don't but deactivating your plugins isn't hard and can sometimes prevent some problems.
It is suggested in the wiki that you do, as well as turning off your board.
if it's from 1.2.x to 1.4.x, yes. If it's 1.4.x to 1.4.x, it says to, but problems arising from active plugins during an upgrade are quite rare.