At this point, most travelers who try to upgrade flights to Europe or other international destinations are used to the co-pay concept; increasingly, for the cheaper fares, the co-pays for those upgrades are as much as the base price of the ticket.

With United Airlines, from San Francisco, the upgrades have also been very difficult to obtain, unless a traveler has elite status with the airline. (Even that is no guarantee.)

While waitlisting an upgrade does tie up miles, at least United fliers haven’t had to book higher level fares just to get on the waitlist, and the co-pay hasn’t been charged until check-in, if the waitlist clears.

Now, with the United/Continental merger, this is changing. If a Mileage Plus member wants to confirm or waitlist an upgrade, the mileage AND the co-pay will be charged at time of the request.

Within the U.S. and Canada, that’s not a big deal, as the co-pays are $75 each way. But anyone trying to upgrade to Europe may have to front up to $550 each way per person for their trip; to Asia and Australia, it costs up to $600.

If the waitlist doesn’t clear, the money is returned. But that means potentially paying credit card interest in the meantime. It’s another possible complication for those who cancel or change their cards.

In addition, United business travelers who get reimbursed for the co-pay when they use miles to upgrade, now may have to either wait to file reimbursements for expense money they have paid out months in advance. If business travelers file for reimbursements at the time of upgrade charge, they may have to deal later with having that charge reimbursed should the upgrade not go through.

On a brighter note, at least with today’s infinitesmal money market and CD interest rates, travelers who have the money to pay off their cards won’t be missing much in terms of income.

Plus, considering that paying a co-pay for now at time of ticketing will push the total outlay for a summer flight to Europe close to $3,000, maybe those fliers who can pay that much upfront might face less competition for upgrades.

Of course this change also comes about when United and Continental are merger systems, and it’s anyone’s guess what will happen to existing waitlists. Oh, and did I mention that Continental’s system is far pickier on exact name matches as far as getting mileage credit?

It’s going to be interesting.