When you upgrade a house, it does not reduce your happiness (and add workers) until you open the present at the end of construction. 

While you can't construct or upgrade 2 buildings at the same time, you can construct while you have a present on your farm.

You don't have to open a present when it is done constructing/upgrading.  One thing many farmers do is upgrade your houses but leave them wrapped up until you need them.  If you have a lull in your upgrades, upgrade a house and save it for later.