Switching out sump

[TriggerFinger]

Hi reefers!
I got a new trigger systems sump to replace the existing one under a 90g tank.

The current sump only has a few pieces of live rock and the return pump. No water in the current sump is going to be reused/saved. I’m making new water to fill the new sump now. I have to change the plumbing...would it be best to first drain the overflow box completely into the old sump?
Any tips on how to make this process go smoothly?

 

[rastareefs]

You will want to drain all the lines and isolate the flow of any water that leaves the DT and goes into the sump. Do you have a valve from your main drain you can close off?