Q. How can Yeshua be High Priest if he isn't descended from Aaron? Wouldn't that prove that the Torah is not for today?

A. The verse you are referring to is Heb 7.12-14. The covenants were for the "natural" children of Israel, that is Abraham, Isaac and Jacob. Non-Jews were once "cut off" from these covenants. Yeshua directed his earthly ministry to the "lost sheep" of the House of Israel, not Gentiles, but things were changing. After the resurrection the good news was to go to "all nations" and that was predicated on faith. Gentiles are brought near to the covenants by the blood of Yeshua (Eph 2.11-22) and are in union with Jewish believers. In Mt 5.17-19 Yeshua did say that the Law and the Prophets would not pass away, but what did he mean. He is talking about the entire Torah and Prophets. Laws are valid but change in application and administration. Laws concerning the Temple, priests, sacrifices are valid but can change in application and who it applies to. For instance, the High Priesthood changed from Abiathar to Zadok, Eli's son's were cut off, Passover changed, all meat was to go to the door of the Tabernacle (Lev 17.3-4) but later it didn't have to be brought there, but to your gates (Dt 12.20-26), the Tamid offering will only be offered in the morning, not the afternoon as before (Ezek 46.13-15), the first Temple was different than the Tabernacle, the second Temple different from the first, Ezekiels Temple is not like any previous Temple, priests can't marry a widow in Ezekiels Temple, Ezekiels Temple will not be in Jerusalem and eventually there will be no Temple at all. There are hundreds of other examples of how things have changed and how things will change. Laws are valid in purpose, but change in administration and application. The Torah is eternal, as Yeshua said in Matt 5.17-19, as the whole, revealed revelation from Genesis to Revelation. None of it will be "discarded" until all is fulfilled but some of it does change in application and administration. In Jer 31 circumcision of the heart is done by God and it is the circumcision required and is the "New Covenant." The problem is that people think the terms Torah and Law mean the same thing, but they don't. Torah is not equated with "law." Torah means "teaching, guidance, instruction." It is the revelation of God's will and intent. The Greek word used for Torah is "nomos" which means law and it is not a correct rendering of Torah. A "law" may not have to be in force at all for it to be still considered Torah (instruction). We can learn and be instructed by the story of Noah's Ark but we don't have to build another one. Torah reveals the heart and character of God as a guide on how to think and act. The Torah had history and genealogies so that's why Torah is not equated with law. So, an eternal Torah would not require a set of eternally set "in force" or continually binding regulations.
God's will is the rule of justice and goodness. Whatever he requires is just and good. Although his creatures are bound by his laws, he himself is not. He is the giver and maintains them. He has a right to dispose of what he wants, when he wants and how he wants by any means he thinks fit, or as I like to put it, it's "his ball." So Heb 7.12-14 does not mean that the Torah has been done away with for all of the above reasons. The Torah is eternal but the "law" on that particular point has changed, as other laws have changed since creation. But, there is scriptural precedent for this change and the change itself is written into the scriptures. It would not be wise to change other laws God has given without it being written by God himself. We are not to add to or detract from that which is written.