I'm playing a game as the Greeks. Fielding armies of Hoplites at 30 strength each. Each Phalanx gets strong from each adjacent Phalanx. They upgrade to a Pikeman at 31 strength and lose their Phalanx ability. Since researching War Summons, I don't appear to be able to build Hoplites anymore. It would have been more beneficial to actually not research the new tech at all. This needs a rework. The most basic stopgap would be a function like Civ. They don't upgrade at all; however, this would violate the historic simulation aspect of the game. Pikeman adopt the unit skin and are the next step.

Most solutions would require a complete rework of combat, I think the most viable solution is a sleight nerf to Hoplites. They are already darn good and Greeks can still make Swordsmen. Heck maybe even some type of tactical ability to boost the strength of adjacent units instead of they themself getting stronger. I have no idea. Right now though, it's a direct downgrade to switch to Pikeman.

Hoplites have a base strength of 27 while pikemen strength is 31. Pikemen cost 20 more industry to build, 200 vs 180 for hoplites, the main weakness of pikemen is their higher upkeep like 20 vs 8 for hoplites. 

However in terms of strength an hoplite would need 4 hoplites next to it to reach pikemen strength, this is possible, it is even possible to go somewhat above pikemen strength with 5 adjacent hoplites but that is a bit extreme in a normal battle. I think pikemen is not a downgrade in terms of pure combat power as they have in general higher combat strength than hoplites and they are more flexible as they don't depend on having many of the same unit next to one another and that can be especially important when fighting in places such as mountains which block how many units you can have next to each other and it make it easier to use other units such as crossbowmen or knights as pikemen strength don't depend on being next to other pikemen.

Narcisse wrote:

Hoplites have 27 Strenght without veterancy. If you upgrade them as Pikemen, your pikemen will have 34 STR. But Hoplites are still a bit too strong , probably

Hoplites are pretty good in terms of balance I would say, they are strong in a specific way, but consider that some units are straight up stronger, or cheaper without any sort of drawbacks or limitations, hoplites seems fine in comparison.