Being that the Medrash Rabba (Vayikra 11:7) explains every mention of the word "v'haya" refers to simcha, how can we understand Bereshis (12:12) where the pasuk begins with v'haya yet continues with a discussion of the dangerous fate of Avraham and Sara entering Mitzrayim?
The Midrash Vayikra Rabbah 11:7 says the following:
(Sources provided by Sefaria.org)
.אתא ר' ישמעאל: עבדה פלגא, כל מקום שנאמר ויהי אין שמחה, והיה אין צרה
Rabbi Yishmael came (and said): Do half 1, any place where it says ויהי there is not Joy, (and here it says) והיה there isn't suffering
(The following is my humble opinion, please take it with a grain of salt)
וַיֹּ֨אמֶר אַבְרָהָ֜ם אֶל־נְעָרָ֗יו שְׁבוּ־לָכֶ֥ם פֹּה֙ עִֽם־הַחֲמ֔וֹר וַאֲנִ֣י וְהַנַּ֔עַר נֵלְכָ֖ה עַד־כֹּ֑ה וְנִֽשְׁתַּחֲוֶ֖ה וְנָשׁ֥וּבָה אֲלֵיכֶֽם׃
Then Abraham said to his servants, “You stay here with the donkey and I and the lad wil go up there and we will worship and return to you"
ונשובה: נִתְנַבֵּא שֶׁיָּשׁוּבוּ שְׁנֵיהֶם
ונשובה AND WE WILL COME BACK — He prophesied that they would both return (Genesis Rabbah 56).
It could be also here that Avram also prophesied that they would both be safe and benefit from this venture.
By 12:12, though it was dangerous for both Avram and Sarai, the result of the venture ultimately lead to Avram amassing much wealth (as demonstrated in 12:16) without Sarai or him being harmed in the venture (see 12:17-20 with Rashi and the Chizkuni on 12:17). That being said, it could be reasonable why the word "והיה" is used.
Hope this is insightful.
1 See context discussed there.