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The thickness- and temperature-dependent irreversible change of morphology and magnetic parameters of 5 and 10 monolayers (ML) of Fe on GaAs(001) are investigated by scanning tunnelling microscopy, low energy electron diffraction, ferromagnetic resonance and magnetometry in ultra-high vacuum. After heating to 550 K (5 ML) or 650 K (10 ML) large flat islands form which have the same vertical layer spacing as the continuous film. The morphological change is reflected in a change of the magnetic anisotropy parameters by a factor of up to 5 in the case of cubic and by a reduction of up to 20% for the overall uniaxial anisotropy contributions. The magnetization of the film did not change after the transformation. These results are explained by the increased number of steps leading to an increase of the shape anisotropy, which in turn reduces the overall uniaxial out-of-plane anisotropy. [ABSTRACT FROM AUTHOR]