This issue affects vanilla Civ & Mac players only.

AlanH identified that the 'upgrade unit' flag is not set for legionaries in this game - although they can be upgraded.

The effect of this is that the unit does not have an upgrade unit button, nor can you use the 'u' shortcut or right-click and choose upgrade unit.

The only way you will be able to upgrade them is to do a mass upgrade. It is recommended that you move any elites that you don't want to upgrade out of the cities, then select a legionary and hit Shift+U to do a mass upgrade.

If this REALLY causes you problems (eg. you "have to" upgrade, and only have enough gold for a few units), then e-mail me your save to gotm@civfanatics.net and i'll fix the issue for you.

... If this REALLY causes you problems (eg. you "have to" upgrade, and only have enough gold for a few units), ...
Similarly, one can move out of cities with Barracks any Legions they don't wish to upgrade or don't have cash to upgrade, then do the Shift-U mass upgrade for the remainder. Assuming you have at least one road for each city so that the non-upgraded units can be moved back if desired.

PTW1.27
GOTM 42. In my game, Warriors have to upgrade to Legionary before then upgrading to Med Inf. Never ever had to upgrade by stages in the past.

That doesn't happen in vanilla. I upgraded one warrior straight to Persian Mercenary (the vanilla version of MDI) - that was what alerted me to the upgrade option for legions in the first place. I doubt if it makes any difference to the overall upgrade cost, but we'll have to look into it.

I was able to upgrade directly from warriors to MIs with PTW 1.27. Could this have anything to do with whether or not you've triggered your Golden Age? :hmm:

I haven't changed anything in the PTW version - so if the upgrading is not working as you expect, then its a "feature", unfortunately.

I think the GA clue might be on the money. Until your GA the upgrade path warrior->legion remains available, even if you have the techs/resources to upgrade legions. Since the dialog that pops up when you hit 'U' isn't equipped to offer you an option on which unit to upgrade to - legion or MDI - it has to offer you legion. So before your GA and after Feudalism you will have to do the double upgrade to turn warrior into MDI