As a senior citizen, can I go back to my old university and take a?
Its unlikely, although I dont know much about the US system. At my UK university once the credits have been used towards a final qualification, they are no longer able to be used towards another final award, therefore only redoing one class would not be enough. You would have to redo the whole degree. If you had enough unused credits you might be able to do this, but it might depend on how long it has been since you did your degree, as most universities I know of have an expiry date on credits (usually 7 years or so), as the material changes so much over time. This might be an issue for you even if it was allowed, as your background knowledge may not be up-to-date enough to cope with the content. The best thing to do is to talk to them to find out what your options are.