It might be more easier to build neutrino based image for dm500+ nfi format then what you are trying.
The old bootloader might not know how to handle the new RAM and Flash chips so while can be an interesting to try, it will not work eventually.
Does dm500+ have a bios where you can select to boot from tftp or NFS ?
For the older dm500 was very easy to use FlashWizard for such images when the image size > flash size.
Good luck