Typically this sort of thing is done by disconnecting 2 of the cathodes not plates. Mesa & Marshall have done this way for instance. Keep in mind, half the # of output tubes, doubles the impedance the amp wants to drive, so your 4 ohm tap becomes 8, 8 ohm tap becomes 16. In spite of this last "fine point" the object is to reduce wattage so most folks don't bother with the impedance change.
Also, the amp's just a little bit less loud when "half power" is done this way. It's not a huge change, won't turn a lion into a kitten.