Be careful about that. Take a look at the Policies page to see the correct type of classes your nation has.

Here is the breakdown:
Absolute Monarchy: Upper = Nobility, Lower = Blue Collar Workers

Republic: Upper = Middle Class, Lower = Blue Collar Workers

Constitutional Monarchy: Upper = Nobility, Lower = Middle Class

So if you are playing a republic, increase the taxes of the 'upper' class to get a revolution to convert to CM.

If you are playing an AM, increase the taxes of the 'lower' class to get a revolution to convert to a republic.

If you are playing a CM, increase the taxes of the 'upper' class to convert to AM.

Increasing the opposite taxes will simply cause a rebellion. i.e. Increasing the taxes for the 'lower' class in a republic will result in a rebellion that replaces your current government with another president + cabinet, etc.