No you cannot change the slider range.
Set the slider itself to, say 0 to 999, then scale it depending on the page.
Code:
// can be done prettier with arrays
p1_s1_min = -50;
p1_s1_max = 50;
p2_s1_min = 0;
p2_s1_max = 127;
pagenum == 1 ? ( s1_min = p1_s1_min; s1_max = p1_s1_max; );
pagenum == 2 ? ( s1_min = p2_s1_min; s1_max = p2_s1_max; );
// then use s1_min and s1_max in your calculations
// I think the calculation is:
s1_value = floor (slider1 / 1000 * (s1_max - s1_min) + s1_min );
Custom GUI? - quite a lot of graphics work but you can have lots of sliders or knobs or buttons or draggable values or ...