changing negative pixel spacing to positive

I have a bug with one application that does not handle negative pixel spacing values. Therefore I would like to change the negative pixel values of my images (geotif) into positive values (while keeping the georeferencing, of course.

I've tried with gdalwarp and gdal_translate but the output still remains in negative values. My workaround was to create a tfw, ten "manually" edit it to get positive Y values and changing the origin to lower left instead of upper left. However, I would like to know if there is an easier (and faster) method to achieve this. If possible using gdal.