Not sure if I am interpreting your question correctly, but if you want to plot a function of the new coordinates, you can use ParametricPlot3D in this way
(I am here using a working example from a very old notebook - it refers to polar coordinates - you might have to invert your transformations)
f[x_, y_] := 3Exp[-x^2 - y^2]
ParametricPlot3D[{
x = r Cos[θ],
y = r Sin[θ],
f[x, y]},
{r, 0, 4}, {θ, 0, 2Pi},
PlotRange -> All, BoxRatios -> {1, 1, .5}, PlotPoints -> {10, 40}]
Otherwise, if you want to plot the transformed grid in the the new coordinate system, I believe you can find the solution to your problem in Xah Lee's Transform2DPlot.m package. This is the man's website http://xahlee.info/. This is the math part of if http://xahlee.info/math_software/mathPrograms.html. Sadly, it appears he is not doing well lately.
EDIT: I see now the package is no longer free to download but requires the payment of a fee.