The day after I started 12/12 lighting I wished my two plants' 3 gallon pots were bigger. 1 bagseed, 1 Kush. I should have just repotted them before 12/12 but I didn't have room till right before switching. I was trying to be sure on id'ing the males (2 of 5 = female).
So after 4 or 5 days of 12/12 (w/ 41d veg) I tried double potting. So far it has come as advertised. At least in this case there's been little to no transplant shock.
This is the basic concept. Just about any bottom container could be used as long as it's at least as big as whatever the plant is already in.
Hated to cut my pots but it's been worth it. They're replaceable. Fingers are not though so be careful.
My pots did not want to be cut despite a new blade.
With the bottom removed and the sliced roots pulled away, it's "planted" right into the other container (which of course has soil in it).
My height restrictions and the containers I have at hand limited me to 1 extra gallon of soil space. It does seem to be helping the plants.
It also adds an interesting new place to water them from.