The command, convert(expr,diff) replaces, where possible, the use of the D operator by the use of the diff function in the expression expr.

$g≔{\mathrm{D}}^{\left(2\right)}\left(y\right)\left(x\right)-a\mathrm{D}\left(z\right)\left(x\right)+\mathrm{D}\left[2\right]\left(f\right)\left(x\,y\right)\:$

$\mathrm{convert}\left(g\,\mathrm{diff}\right)$

$\frac{{\ⅆ}^2}{\ⅆx^2}y\left(x\right)-a\left(\frac{\ⅆ}{\ⅆx}z\left(x\right)\right)+\frac{\partial }{\partial y}f\left(x\,y\right)$