Primary and secondary laws

I've recently been reading up on Chinese history, and I encountered the following passage regarding laws in the Tang dynasty. From A Brief History of Chinese and Japanese Civilizations,

This got me thinking about chukim and mishpatim. Yes, we know the traditional (midrashic?) explanation that chukim are decrees with no explanation while mishpatim are laws to which we might know the reason.

But how about this for a distinction? A chok is something that is eternal, for all time. Thus:
שמות פרק ל

• פסוק כ"א: וְרָחֲצוּ יְדֵיהֶם וְרַגְלֵיהֶם, וְלֹא יָמֻתוּ; וְהָיְתָה לָהֶם חָק-עוֹלָם לוֹ וּלְזַרְעוֹ, לְדֹרֹתָם.  {פ}

A chok-olam is an everlasting portion. And similarly, chukat olam which occurs 16 times, including:

שמות פרק יב

• פסוק י"ד: וְהָיָה הַיּוֹם הַזֶּה לָכֶם לְזִכָּרוֹן, וְחַגֹּתֶם אֹתוֹ חַג לַה':  לְדֹרֹתֵיכֶם, חֻקַּת עוֹלָם תְּחָגֻּהוּ. 

How shall we account for mishpat as a changing law? Isn't there a prohibition of bal tigra and bal tosif? We could answer that the Torah itself accounts for different circumstances. There are halachot which hold during the time of heter bamos and during the issur bamos, when Yovel is practiced and when it is not, when people are willing to lend close to shmita and when they are not.

Of course, this would require a cataloging of chukim and mishpatim and see whether this distinction holds true...