Yes product upgrade change is possible from Walmart storecard to Discover card. The requirements are you need to have establish at least 6 months of payment history. I'm going to do this sometime in November. Either you can do the upgrade change or apply for just a regular Discover card at Discover website.
The Walmart Discover card are issued by GEMB Money Bank licensed by Discover Financial Services Network.
Mix really isn't an issue. I used to have all the walmart, best buy, target and so ons... they got my score up initially but I closed them out when I started getting CL's over 5K, 10K, and 15K. The only benefit they have is that they're easier to apply for when you're starting out or rebuilding due to more leniant guidelines for approval and their outrageous APR's.
Just my $0.02