@Tinytuttle I started doing this a long time ago after watching @BogSeeds transplanting method back in the day. When I grew in soil with limited space in 3 gallon pots could easily add a significant area above the surface greatly increasing root space in the same pot. In coco however it washes out too much IMO to xplant up too high. I can’t remember BOG’s exact method but in soil I would start in RW or Peat pucks, go to a 24oz solo cup, 1 gallon pot to final 3 gallon leaving 1/3-1/2 the original container above ground.
The biggest benefit for me and why I continue to do it a smaller degree, is it really helps in the constant fungus gnat war. The root ball is on top pretty much always staying dry as well as all those nice barky roots on top the larvae can’t harm. If I see adults I bottom water until they are gone, spray mist the tops with Neem Oil/peppermint soap and all I’ve had to deal with is the occasional adults that make their way in that can’t make it past the first generation If gnats weren’t an issue I probably wouldn’t do it, but they are a big enough issue that I’ve lost entire grows/irreplaceable genetics not having preventative steps in place ahead of time.
just dug this up from the old GrowFAQ… BOG's double potting technique