This is actually the second time this has happened to me, but I don't remember what I did last time (nor can I find the original forum post that helped then).
I'm using Quicken Mac 2007. My 401(k) account, previously properly set up, has changed, seemingly irrevocably, to a Checking account. When I click "Edit" and try to change the account type, the 401(k) option is grayed out. I've got years of data stored in this account, so I am really hoping there's a way to salvage this data file!