replacing carpet in the only room in the house that isn't engineered hardwood. existing HW flooring is long discontinued. i have found matching flooring that is 1/8 thicker than existing. Can i plane 1/8 inch from the bottom side of 1/2 inch engineered flooring?

You will probably not have the warrantee if you plane anything off the bottom of engineered flooring.
Check where the new flooring will meet the old with a straight-edge to see if you could plane or grind the sub-floor for a couple of rows in the doorway. An electric hand plane will take off an eighth inch quickly. An edger with 16 grit can do it too. A gentle slope will not be as much of a wear area, but you could do it in one row, too. Glue the planks to help keep the sub-floor strong. Set nails before you start planing. A chisel will get the ends where the machines can't reach.
You could plane the bottom of one row and glue it with a weight on it. It depends on how the sub-floor looks whether you want to have a gentle slope or a bevel.

Thanks. Good thoughts. Only issue is it’s not a doorway separating the two spaces. It’s an open area so the current hardwood meets the carpet along about 18+ linear ft.

An area rug could make the height difference less noticeable after you use the electric plane or edger to make a gentle slope from the old floor.