All destinations have different rules.

In general, a full "Y" ticket has no restrictions on min stay, advance purchase and no change fee. Some "M" class has no change fee. Most of the lower classes (N,L,Q,V,H,T,B...) usually carry a change fee ranging from $100 to $200+.

Also, you can only make changes to your inbound trip only. Changing the outbound would mean you need an entirely new ticket in most cases unless you buy up to the next highest fare.