I am booking several Star Alliance multi-carrier tickets to Asia for my family. UA says that once you begin travel, there are no changes allowed at all - for dates, times, standbys or anything.

What has been peoples experience actually travelling. It seems like if you showed up at the airport a day early and there was space that they would let you go. Is this not the case? Does it matter if it is UA vs. Thai vs. Singapore Air?

Thanks so much!