EVA currently has some excellent fares from USA to SEAsia. Problem is, these really low prices allow a maximum stay of 3 months. I am planning a longer trip, making the best fares unavailable, and the next class is significantly more expensive. So my thoughts were to book a roundtrip ticket using the lowest fare class (i.e. return within 3 months), then later change the date of the return leg (and pay 50 USD fee + fare difference). Does anyone know, as far as calculating the fare difference, would I likely be able to stay within the low fares* or would I automatically get kicked up to a higher fare class?

*I understand there is no guarantee the low fares will continue to be available, but EVA has been running these specials consistently for many months now.

Thanks for any replies.

ps- other carriers don't have the same 3-month restriction but I like EVA, with a 9-wide config in economy, wifi the whole way, and friendly staff.