I haven't gotten much hands-on experience with 3/4UC Monty, but from a pure mechanics standpoint I agree that the Eagle's free worker promotion is much more useful coming in the Classical era than Medieval. Having said that, given the history of the units I also agree that Jaguars upgrading directly into Eagles just makes too much sense for it to work any other way. However, to also play the devil's advocate, I understand the sand in the vaseline that is having Jaguars needing to upgrade into Spearmen before becoming Eagles if Eagles became Pikemen replacements. And then there is the whole historical inaccuracy of having Jaguars be a Warrior replacement when we know that their culture thrived from at least before the 1200s through the 1500s. Jaguars should really be a Spearmen or Swordsmen replacement with Eagles being their direct upgrade at Pikemen/Longswords, respectively. Buuuuut I also recognize that part of the pizzazz of the Aztec Uniques is having a special unit right from the get-go.
Personally I lean towards historical parity, but I completely understand and agree that it's game mechanics that should take priority, and that we could use more Warrior Uniques and less Classical/Medieval Uniques.
But perhaps there is a happy medium that can be achieved? Here is my more radical suggestion: Swap the unique promotions of the Jaguar and Eagle and move the unit line into the Classical > Medieval Era stretch of the timeline.
Specifically:
- Alter UA to add +33% CS

when fighting in jungle/forest promotion to all Aztec melee units.
- Jaguar becomes a Swordsman replacement w/ +2 CS

and Captives of War (Loses Woodsman & +25 Healing on Kill); Requires no Iron; Becomes available at Mathematics (or Masonry?) instead of Iron Working and obsoletes at Theology
- Eagle becomes a Longswordsman replacement w/ +3 CS

, Sentry, and +25 Healing on Kill; Requires no Iron; Becomes available at Theology instead of Steel and obsoletes at Rifling
This does a few things:
- Retains Aztec bonus to jungle/forest combat right from the start of the game.
- Moves Jaguar and Eagle warriors into closer parity with their historical appearances (and makes them direct upgrades into each other without creating a unit with OP promotions).
- Differentiates Aztec UUs from Iroquois UUs by making Woodsman unique to Iroquois Mohawks and giving Aztec UUs more base CS

.
- Keeps Captives of War promotion available in the early Classical era where its utility remains intact.
- Ties Aztec UUs to technologies that are not usually associated with combat but make sense given the other tech unlocks for their UB and UNW, giving the Aztec one more unique aspect to their mechanical gameplay (can focus more on "upper-middle" of tech tree).
- Keeps the Jaguar from stepping on the toes of other Swordman replacements and the Eagle from stepping on the toes of other Longswordsman replacements.
- Allows for Eagle to immediately obsolete Jaguar, which is fine because all that is lost from not upgrading a Jaguar to an Eagle is Captives of War (which is mostly irrelevant by this point). So you can have a few elite Eagles upgraded from your existing Jaguars that retain Captives of War, but otherwise you're forced to produce fresh Eagle Warriors once Huey Teocalli is available (which feeds into Huey gifting freshly produced units with bonus XP).
Thoughts? Is this way too much?