To drill or not to drill?

[Kmarrs]

I recently setup a new 40 gallon tank. I bought it used with a whole pile of equipment but it also came with an extra 20 gallon long that I intended to run as a frag tank. The display tank currently has a "built in" style sump that runs along the back a lot like a red sea max and works quite well. I would like to increase my total tank volume and also have a frag tank running in-line..

My plans are to drill two holes in the bottom of the display tank to create a herbie style overflow. drilling where the return pump currently sits because, there is enough room. I am looking for your opinions on whether or not to drain the entire display or if i could get away with drilling the sump section of the tank while water still sits on the other side. This all makes a pretty big difference on the amount of time it would take me to drain and move livestock versus just drilling and plumbing.

Thanks in advance.