AC's fares in Europe are sometimes a bit of a patchwork quilt for reasons only they and regulators can understand. Having said that, AC.com is even more of a patchwork quilt. You might want to try PHONING AC and asking them what the M fare is on the segment of interest to you. Just because AC.com doesn't show it does not necessarily mean it is not available.

Having said that, I believe you are correct that if it's I0 for all the segments of interest to you, save your money and book in H class since you won't get the instant upgrade anyway.

My two cents.