Or HaChaim on Numbers 3:45/אור החיים על במדבר ג׳:מ״ה

1 והיו לי הלוים אני השם, "and the Levites will belong to Me I am the Lord." The reason the Torah added the words "I am the Lord," is to tell us that although according to Yalkut Shimoni 364 there will come a time when the firstborn will once again be performing the priestly functions in the Holy Temple, the Levites will not therefore be demoted and cease to belong to G'd. [I have not foumd a reference in the Yalkut to the firstborn being reinstated as priests in the future. Ed.] The very expression והיו לי indicates that just as G'd's name is eternal so is the appointment of the Levites to their task.