I am sure AA would like to refurbish and replace thier fleet, but with the cost of fuel (and the announcement today they will have to pay an $1b this year than originally calculated), AA is trying to just survive.

L, M, H, K, W, fares all provide for a lot of the flexibility of the Y fares (no change fees etc) without the massive upfront costs

All of the fares (expect Y) mentioned above do have change fees.