This problem has happend to me as well. Just as early as last week, I had 7 albums as well as about 181 individual songs that I was going to upgrade with iTunes plus. As of checking in iTunes last night, the only option I had to upgrade were the entire ablums, the individual songs had disappeared. Since this is clearly not an isolated incident, I think it would behoove Apple to either do some investigating or offer an explanation as to why this has occurred.
If it's a change in policy, shame on Apple. If it's a glitch and they can't understand what the problem is, they need to address qualifications for hiring support personnel because I read the problems stated clearly and I'm currently experiencing the same problem.
I'd hate to know that I missed out on this opportunity by a week! I use SD cars for the MMI system in my Audi so that I don't have to plug my iPhone into the AMI interface in the glovebox. Real pain in the rear having DMR protected songs that can't be played.