Best way to move/open up existing holes in 2x4 for proper drain slope?

[DIY_Novice]

I have existing holes for bathroom vanity plumbing in my 2x4 framed wall, however they are not 1/4" per ft slope, and I'd like them to be. Does anyone have a recommendation on how to "open" these holes upwards (effectively, create an oval)? A jigsaw or sawzall won't fit - something better than sanding?. I know a few tricks to opening the diameter on an existing hole with a hole saw, but not how to move re-drill with a hole saw at a 1/4" above the existing hole. I don't want to go with larger diameter because it's already close in terms of the allowable cross section cut-out. Anybody have a good tip/trick?

Need to open up the black line hole to the red line hole.

 

[Reach4]

Cut a hole in a piece of scrap wood.

Position that hole over the old hole, but with the offset you want.

Nail that scrap of wood onto place over the old hole. Use the hole in the scrap to guide the hole saw.

 

[Sponsor]

 

[John Gayewski]

You can also just screw a strip of 1/4" plywood over the hole and drill your new hole through it.

 

[jbot]

Both ways posted above work but you can also just use a hole saw of same size to oval out the hole. Take your piglet bit out first so you don’t break it off.