I doubt the upgrade is possible not sure the version in the Acer is MXM? 260 is MXM3 not sure about 240? But if the size difference mentioned above is the 40nm vs 55nm that is not a difference that has any effect on if it fits.
its the actual physical size of the card that differs. the 260m is a bit larger than the 240m. even if they were compatible in every other way the physical card would not fit in the space that is provided for the 240m.