Ok, then what I said earlier (DP to VGA needing a power source) might be the correct answer then. Are you able to buy an active adapter if you can't find a solution through drivers?
And another, a bit more terrifying thought, the DP port, or the card itself, might be faulty. Do you have a way to test the other ports to see the card is actually working?
And just to clarify: if you install the new drivers using the old card, it will install the old card drivers instead. AMD and nVidia bundle a *lot* of card drivers into the same bundle, but when installing in a fine grained way, they use different files underneath. That means (for short), you cannot install the drivers for the new video card using the old card, even when they are of the same brand.
Cheers!