Many people that subscribe to a group, or belief system that state the laws of the Torah of Moses are still in effect because they were declared to be "eternal," several times in the Torah and ought to be kept by Christians (or atleast Christians of Jewish Ancestry) have no problem admitting that the Levitical Priesthood was changed and abolished because of the Epistle to the Hebrews.
However, the Old Testament itself DOES call the Levitical priesthood "eternal," as we can see here with the incident of Phineas slaying the Israelite and the Midian woman.
וְהָיְתָה לּוֹ וּלְזַרְעוֹ אַחֲרָיו, בְּרִית כְּהֻנַּת עוֹלָם--תַּחַת, אֲשֶׁר קִנֵּא לֵאלֹהָיו, וַיְכַפֵּר, עַל-בְּנֵי יִשְׂרָאֵל.
and it shall be unto him, and to his seed after him, the covenant of an everlasting priesthood; because he was jealous for his God, and made atonement for the children of Israel.' -Numbers 25:13
As we can see the priesthood is here refered to as כְּהֻנַּת עוֹלָם k'hunnat olam (kehunat olam).
After doing some searching I then found that the priesthood is described as eternal elsewhere too
וּמָשַׁחְתָּ אֹתָם, כַּאֲשֶׁר מָשַׁחְתָּ אֶת-אֲבִיהֶם, וְכִהֲנוּ, לִי; וְהָיְתָה לִהְיֹת לָהֶם מָשְׁחָתָם, לִכְהֻנַּת עוֹלָם--לְדֹרֹתָם.
And thou shalt anoint them, as thou didst anoint their father, that they may minister unto Me in the priest's office; and their anointing shall be to them for an everlasting priesthood [lich'hunnat olam] throughout their generations.' -Exodus 40:15
God said He would have His Eyes and Heart in the Temple FOREVER:
וְעַתָּה, בָּחַרְתִּי וְהִקְדַּשְׁתִּי אֶת-הַבַּיִת הַזֶּה,
לִהְיוֹת-שְׁמִי שָׁם, עַד-עוֹלָם; וְהָיוּ עֵינַי וְלִבִּי שָׁם,
And now, I have chosen and consecrated this House that My name be there
forever [ad-olam, עַד-עוֹלָם ], and My eyes and heart will be there at all times.--2 Chronicles 7:16
Now, how could He dwell there literally forever if there was no temple for years at a time, and even today there has been no Temple for over 1900 years! God knew the future, "forever" just means for a long time!
Lamentation 3:6 in just about EVERY bible translation translate OLAM as "long"
בְּמַחֲשַׁכִּים הוֹשִׁיבַנִי, כְּמֵתֵי עוֹלָם
He hath made me to dwell in dark places, as those that have been long [olam, עוֹלָם] dead.
Psalm 143:3, similarly translates OLAM as "long"
הוֹשִׁבַנִי בְמַחֲשַׁכִּים, כְּמֵתֵי עוֹלָם.
he hath made me to dwell in darkness, as those that have been long [olam, עוֹלָם ] dead.--Psalm 143:3
It is also interesting to note that King David said only the Levites were to carry the ark and serve before God 'forever'.
אָז, אָמַר דָּוִיד, לֹא לָשֵׂאת אֶת-אֲרוֹן הָאֱלֹהִים, כִּי אִם-הַלְוִיִּם: כִּי-בָם בָּחַר יְהוָה, לָשֵׂאת אֶת-אֲרוֹן יְהוָה וּלְשָׁרְתוֹ--עַד-עוֹלָם.
Then David said, "It is not proper to carry the Ark of God except the Levites, for the Lord chose them to carry the Ark of God and to serve Him forever [olam]. "-1 Chronicles 15:2
So, even though, the Torah refers to the priesthood as being for "olam" this was not understood by the New Testament writers to mean it would never be abolished, as we see in Hebrews 7
When there is a change of priesthood, there is necessarily a change of law as well. --Hebrews 7:12
So, as we can see, since the Levitical priesthood of the Old Covenant was abolished as the book of Hebrews tells us, and the fact that the Levites no longer "serve" God forever, that olam, does not necessarily have to mean literally FOREVER.