The phrase de iure (or de jure) means literally "by law".
Law can mean many kinds of formal decisions in different contexts.
The phrase de facto is similar, but harder to render concisely in idiomatic English.
I would translate it as "by actual events".
For natural-sounding use of these, it is important to keep the preposition "by" in mind; the sentence should make sense when you replace de iure by "by law".
For example, if you have been appointed a leader, you are a leader de iure (leader by formal appointment), but if you have taken up a leading role without formal promotion, you might be a leader de facto (leader by actual events).
Your saying does not fit this structure.
Replace the Latin phrases by my translations (or better ones), and you will see an issue: You cannot build "for by law".
To make the saying work well, either "for" or "by" has to go.
You seem to be treating de iure and de facto as nouns, whereas they are adverbs (modifiers that express a method).
I have a couple of replacement options to suggest:
Build for ius, change for factum.
While this would be a grammatically correct way to remove the preposition de, it would be hard for many people to parse.
Sticking to the well known de iure and de facto is better.
Build de iure, change de facto.
Replacing the Latin phrases with their translations leads to this meaning: "Build by law, change by actual events".
This is grammatically valid, but I still see a semantic issue.
The phrases de iure and de facto usually describe how something happens (roughly: formally vs. informally), not why.
[Good code is] built de iure and changed de facto.
The difference to the previous one is that the coding is described afterwards.
This feels more appropriate, but it does still feel somewhat unnatural to me.
Build for the ideal, change for the real.
As mentioned above, I don't think de iure and de facto are the right kind of modifiers for this purpose.
Instead, I would rephrase with other words.
Also, using plain English words helps get the message across to those less familiar with Latin phrases.
There are many possible variations, like changing "change" to "adjust" and "the real" to "reality".
I strongly recommend the fourth one.
While de iure and de facto are in roughly the right direction, I do not find them good fits to this use.