That's not a problem at all – the re-modernization coefficient can be adjusted in DefaultStats by 100%. Then, when upgrading to a more expensive creature, you will pay 100% of the difference, and there is no abuse.

No, changing this setting will not make you - PAY 100% OF THE DIFFERENCE in upgrade costs, but rather - pay an amount equal to the initial cost of the creatures. Or any other amount that has been adjusted in the settings, but not - the difference in prices.

Added 16 minutes ago

Given that opinions on this mod's idea are roughly split, I would do the following: leave the prices for the 1st upgrade of creatures (and the prices for non-upgraded creatures) as they are, and create a "resource-based" price option for the 2nd (alternative) upgrade of creatures (according to the scheme: we add resources to the hiring cost, but the price itself is slightly lower than the original).
I think it could work like that. .... In general, I liked the author's idea.

Unfortunately, this is not possible. In order to make this possible, the SECOND upgrade must have the same mechanics as the regular upgrade (from simple creatures to improved ones); then everything would work correctly, and only the price difference would be paid. But in the current case, the game simply does not accept upgrades of different costs and is not able to handle them correctly.

As a result, in each new game, you can act according to the circumstances: if resources are scarce, buy the 1st version of upgraded creatures; if there is an excess, buy the 2nd version.

Yes, but, in my opinion, such a choice would become too - economically driven, and as a result, in the vast majority of cases, the player would tend to spend their GOLD reserves in an attempt to make the game easier for themselves, thereby depriving themselves of the pleasure of having troops that are more tactically suitable for the given situation.
However, setting 2 upgrades to different TYPES of resources seems quite interesting; in this case, the player would no longer be able to "escape the problems" by buying troops for gold by choosing the first upgrade, but at the same time, he would still have some choice, where to get out of the situation, it would be enough to correctly prioritize the capture of mines. Unfortunately, this is also not feasible.
Added 12 minutes ago
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The thing is that upgraded creatures have abilities. And this, in turn, provides variety in tactics. This mod cuts down on this variety. The idea itself is not bad, but it still needs a lot of work.

I'll polish up my creation a little more (it's 5 years old: D), but there won't be any fundamental differences.