You might have guessed based on recent questions of this sort that I would turn to my favorite Etymological Dictionary of Biblical Hebrew, from the writings of Rav Hirsch.
On page 108, there is an entry for the root יצב, which he translates as “erect; stand firm independently.” The applications he brings are: standing firm and upright (as in our passuk, נצבים); to erect (ויצב שם מזבח, Bereishis 33:20); to endure (יצב גבולות עמים, Devarim 32:8); a memorial (מצבה, Devarim 16:22); a pillar (נציב מלח, Bereishis 19:26); a military position (ויצב מצב פלישתים, Shmuel 1:13:23); and the stem of a tree (בשלכת מצבת בם, Yeshaya 6:13).
Compare this to the root עמד on page 187, translated as “stand in preparation to act.” Some applications parallel those of יצב above: enduring (ויכלת עמוד in Shemos 18:23), a pillar (עמוד ענן, Shemos 13:21), a station (והדפיתיך ממצבך וממעמדך יהרסך, Yeshaya 22:19; note that יצב is also used here), and ordinary standing (ולא עמד איש אתו, Bereishis 45:1); then there are those which are very different: stopping (ותמיד מלדת, Bereishis 29:35), sustaining (למען יעמדו ימים רבים, Yirmiya 32:14), raising (ויעמד רוח סערה, Tehillim 107:25), and receiving strong support (אם יהיה אלקים עמדי, Bereishis 28:20).
In other words: יצב means standing firmly planted, with no intention of going anywhere; עמד means standing at ready, preparing to act.
I think it is clear that the word נצבים in our parshah comes to indicate this when you consider that Moshe is passing them into a covenant. While a covenant is a commitment for the future, it’s more precisely a state of resolve - that is, one who is accepting a covenant is standing firm in resolution of accepting the agreement, rather than standing at the ready to fulfill the agreement. (Don’t believe me? The same dictionary on page 32 defines ברת as “to separate, highlight.”)
Rashi’s Midrashic explanation (placed after his comments to v. 12) give another layer of meaning to this choice of words:
עַד כַּאן פֵּרַשְׁתִּי פְּשׁוּטָה שֶׁל פָּרָשָׁה. וּמִדְרַשׁ אַגָּדָה: לָמָה נִסְמְכָה פָרָשַׁת אַתֶּם נִצָּבִים לַקְּלָלוֹת? לְפִי שֶׁשָּׁמְעוּ יִשְׂרָאֵל מֵאָה קְלָלוֹת חָסֵר שְׁתַּיִם חוּץ מִמ"ט שֶׁבְּתוֹרַת כֹּהֲנִים, הוֹרִיקוּ פְנֵיהֶם וְאָמְרוּ מִי יוּכַל לַעֲמוֹד בָּאֵלּוּ?! הִתְחִיל מֹשֶׁה לְפַיְּסָם — אתם נצבים היום, הַרְבֵּה הִכְעַסְתֶּם לַמָּקוֹם וְלֹא עָשָׂה אִתְּכֶם כְּלָיָה וַהֲרֵי אַתֶּם קַיָּמִים לְפָנָיו:
Until here have I explained the simple meaning of the passuk. This is the Midrash Aggadah: Why was Nitzavim placed next to the curses? For when Yisrael heard the 98 curses, plus the 49 from Sefer Vayikra, their faces became sickly, and they said, “Who can withstand this?!” Moshe began to appease them: “You are standing today” - you have angered Hashem very much, but He has not destroyed you; behold, you are still extant before Him!
In this explanation (my translation), it’s much clearer that the intention is to underline their standing firm today, not their outlook onto tomorrow.