The Mishnah at the end of Eduyos tells us that Eliyahu ha'Navi will determine whose lineage is reliable and whose is not. He will identify the true Kohanim who are fit to serve in the Mikdash.

(A Mesorah from the Arizal tells us that Bechoros will also be permitted to serve. This is alluded to in the Sofei Teivos of the verse "Tzadik ka'Tamar Yifrach", which read "Korach." Korach (and the Bechoros) will be given a chance to serve in the Mikdash in the future.)

After Techiyas ha'Mesim, of course, there will be many more Kohanim. All of the Kohanim in history who do not fit into the categories of the Mishnah in Sanhedrin 90a (e.g. one does not believe in Techiyas ha'Mesim etc.) will come back to life, and will be given the opportunity to serve in the Mikdash.

Be well,

M. Kornfeld