Will Demo images for IMXQSB work for IMX53 SABRE.?
I would say no.
You need a different bootloader for QSB and SABRE. But you can reuse uImage and rootfs.
In order to use the same uramdisk you need to make sure it has the same SDCard configuration (for example, SABRE would take rootfs from SD1, and QSB would take from SD0, I do not remember it there is a difference, so, double check it)
So= you may reuse some itens from a demo image, but a pure demo image will not fit other board