This is what I’d do to change a mask … (this works fine for me in an app that’s been in the app store since masks came out. ;-))
local mask1 = graphics.newMask( t.maskFile1 )
t.parent.touchBkgd:setMask( mask1 )
-- ... stuff happens.
local mask2 = graphics.newMask( t.maskFile2 )
t.parent.touchBkgd:setMask( mask2 )
So, I dunno what you’re doing exactly that it’s not working for you – but something is up if you say you’re removing the original image, then creating a new image and then adding a new mask … I mean … your original mask isn’t associated to the new image at all… that’s like saying that any image you add inherits the mask. ?? I think you’ve got something else going on. 
Anyway … In the example above, it changes the mask on the touchable background from mask 1 to mask 2.
(And my x/y change suggestion was only for use of actually removing a mask, not replacing one as that works fine for me just by adding the new mask.)
Good luck! 
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