I would suggest the (paraphrased) Nancy Reagan approach:

JUST DON'T TRY IT!!!

I changed dates and destinations for a flight that I took last month. All looked fine (I was even upgraded five days out) until I tried online checkin...no option to check in. Hmmmmm.

Turns out that the system had never ticketed the reservation, and it took over an hour and multiple supervisor contacts by the phone agent to get the ticket done. They even had to ask me what I thought the fare was, since the system didn't have it!!! (I said 40 bucks, but they didn't buy it).

What a near nightmare. The agent said that I should ALWAYS call an agent in the future if I need to change flights.