Unfortunately at the moment it isn't a simple change but also not that complicated either..
Using a simple text editor you should be able to edit these files to do what you want.
In Folder :
\Interface\AddOns\nUI\Layouts\Default\HUDLayouts
Then one of these folders depending on which hud layout you want it changed for :
NoHUD, NoBars, HealthPower, PlayerTarget
Using PlayerTarget as an example open up the following file in that folder :
nUI_HUDSkin_PlayerTarget_Castbar.lua
The section I believe you want to look at is :
Code:
["Casting"] =
{
anchor =
{
anchor_pt = "BOTTOM",
relative_to = nUI_HUDLAYOUT_PLAYERTARGET.."Bottom",
relative_pt = "TOP",
xOfs = 0,
yOfs = 130,
},
Change
xOfs for left/right movement and
yOfs for up/down movement. You can make this change while in game or out. Just relog or reload the ui with /nui rl and it will reflect your change. If you are nervous of making changes, make a copy of the file to fall back to if something goes wrong