in fall.. why not just dig the plant out.. add you soil/media and throw it back in ...

in the meantime.. for the heat of summer.. just fill the void with some bark mulch or something ...

if you insist.. add an inch now.. an inch in late fall .. and an inch next spring ... taking settling into account ....

do it in stages.. rather than burying it.. shocking it ...

ken

I would basically do as if I were transplanting it elsewhere. Water the soil deeply. Extract the plant with as many roots as you can and temporarily place it in an area with full but bright shade & few/no winds. Add soil to raise the level of the soil and plant it back in the same spot.