Upscaled dvds generally won't look as good as blu rays, but the difference will depend on the quality of the blu ray transfer and that of the dvd as well as the upscaling capabilities of your blu ray player and/or tv.
Just because a dvd is upscaled to 1080p does not produce HD picture quality.
Whether or not you choose to updgrade is a personal decision, which will no doubt depend on how happy you are with the quality of the dvd in question on your exising set up, how much you like the content and whether you think that re-purchasing the blu ray is worth it at the price. A little online research will help you figure out the quality of the transfer.
I won't be upgrading any dvds, as I am happy enough with how my dvds look. Additionally, I bought a lot of my films for between £15-20 upon release and have only watched them once or twice (except for absolute favourites which I may have seen 3 times). On that basis, I'm not sure it's worth it for me to pay an additional £10-20 to get the blu ray.
As I say though, it's a matter of personal choice.