Yes. The difference is that a "restricted" M fare -- and I have seen them -- will make the ticket non-refundable and cause a $100 change fee and possible re-faring, at least in the US for voluntary domestic res. changes. There is no difference if, for example, you book an "M" fare to use an upgrade or a mileage promotion that requires an M (or other) fares. Then, any "M" fare should be OK.
How can you tell if the "M" is restricted? The fare rules on ual.com should show that. Also, use of the letter "N" within the M fare code is a giveaway. (Ex. M7NEZ) would probably be "restricted" but MUA might not. However, the actual fare rules would control.

[This message has been edited by Ocn Vw 1K (edited 05-15-2003).]