Originally posted by doconner:
...just to earn miles so that at some later time you get to sit in first class for free rather than paying for the upgrade?
Not all airlines will allow you to pay for an upgrade (other than buying a full first-class ticket.) For example, the only way that you can get into FC on Continental is with miles or status. The higher your status, the better your chances of getting upgraded.
You'll find, though some may disagree, that there are no stupid questions in FT. Feel free to ask. (Although, getting to know the search function doesn't hurt, either.)
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Bob