What does the Torah mean when it writes "Me'eis b'nei Yisrael"?
Rashi: It means that Yisrael would hire the Levi'im 1 to serve in the Beis-Hamikdash instead of them.
And what does it mean when it writes "Tachas Kol Bechor ... "?
Rashi: It means that they merited this honor - that Hashem chose them - on account of the firstborn who were originally destined to serve, but who forfeited that right when they worshiped the Eigel, whereas they did not.
Seforno: It means that the Levi'im took over from the Bechoros by means of redemption (as the continuation of the Parshah will explain).