Modding question about national hero's

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AFAIK you cannot mod the "link" between a nation/theme and its heroes unit id.

*But* you can use the heroes "slot" to change them to whatever you wish, as proposed with #clear etc..

So in the end you can change them completely, but the number of nation heroes can't change.
Is that clear and does it answer your question ?

BTW it's the same thing for thematic or site-produced units (capitol units for example) : themes and sites produce given unit ids, but you can change the units.