import bpy
import bmesh

modif = 1.5

me = bpy.context.object.data
obj = bpy.context.object
shapekeynames = []
shapekeys = me.shape_keys
for shapekey in shapekeys.key_blocks:
    shapekeynames.append(shapekey.name)

for shapekeyname in shapekeynames:
    if shapekeyname == 'Basis':
        continue

    shapekeys.key_blocks[shapekeyname].slider_max = modif
    shapekeys.key_blocks[shapekeyname].value = modif
    # apply new 1.5x shape as mix, append old name with extra modifier
    newShapeKey = obj.shape_key_add(shapekeyname + '_NEW', True)

    shapekeys.key_blocks[shapekeyname].value = 0
    # delete old shape
    obj.shape_key_remove(shapekeys.key_blocks[shapekeyname])
    # rename new shape to old name
    shapekeys.key_blocks[shapekeyname + '_NEW'].name = shapekeyname