Can anyone tell me what is the lowest fare class on Delta that will allow changes? I'm not talking about free changes. I would like to book flights for my kids this summer and we are 99% of their summer schedule, so I want to lock in the great fare now. On the off chance something changes, I don't want to be stuck with a ticket where the return date can't be changed. I know changes can cost $100 but a recent phone call to Delta said some fares don't allow any change at all - I'd like to avoid those fare classes

Any advice?

Thanks