United has long offered a program of earning or purchasing e-500 mile upgrades. As an elite member, the upgrades come complimentary after a certain amounts of miles are flown. One can also purchase packages of e-upgrades. Then, buy a coach ticket, apply for an upgrade to the next class of service, and as (if) seats are available people are upgraded.
People unable to use the e-upgrades because no upgraded seats become available, or they simply don't fly enough to use them up could at least look forward to those e-500 mile upgrades to be converted to regular United miles. This occurred 1 year from the date the e-500 mile upgrades were issued.
New Policy: United states the 500 mile upgrades will no longer be converted to miles.
UA flyers who fly often have accrued many upgrades and will take a big loss with this new policy. Infrequent travelers may not even notice the change. Others will fall somewhere in between. United however made this policy change quietly (small blurb at the bottom of a page on their website) and with very little notice. Many have criticized United not simply for this policy change, but the method they used to do so.
Here is the link discussing their e-500 mile upgrade program; at the bottom of the page is their new policy.
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Update: 6/3/07 United Airlines has modified this policy and in certain cases Premier Exec flyers and higher can ask to have upgrades coverted to miles. Low or no status UA flyers are still out'a luck. Thumbs down to United!