Deutéronome 26:3

And you shall put crowns upon the baskets, hampers, and paper cases, and bring them to the priest appointed to be the chief priest in those days, and shall say to him: We acknowledge this day before the Lord thy God that we have come into the land which the Lord sware unto our fathers to give us.

The phrase in those days means that the obligation is in force as long as there is a High Kohen.

I declare out loud, so that the young ones will hear. The verb may also denote “to acknowledge” — i.e., thanking God that He fulfilled his promise — and this interpretation is supported by the presence of the fruit.

AND THOU SHALT COME UNTO THE PRIEST THAT SHALL BE IN THOSE DAYS — to the priest that shall be there in those days, for first-fruits, like all offerings, are given to the men of the [priestly] Division [who are assigned to the Sanctuary for a week at a time], but he may not bring along a priest from his city in order to offer him his first-fruits.
Now, Rashi wrote: “You have none else than the priest who lives in your days as he is.” But I have not understood this, for in the case of an elder [of the Sanhedrin] it is proper to say [and thou shalt come …] unto the judge that shall be in those days, meaning that even though he is not as great and wise as the early judges in the ages which were before us, still he [the elder] is obligated to hearken to him [the judge], for “Jephtah in his generation is as Samuel in his generation.” But when bringing first-fruits, to whom shall he bring them if not to the priest that shall be in his days? Now, I have also seen the following text in the Sifre: 298. “And thou shalt come unto the priest that shall be in those days. This is what Rabbi Yosei the Galilean says, And would it occur to you to come to the priest that is not in your days? Rather, [I must say that the verse requires one to come to] the priest who is considered by you to be established and fit in those days. Kinsmen who have ceased to be kinsmen are qualified. And so it is also said, Say not thou: ‘How was it that the former days were better than these?’ ” thus refers to “the judges of the former days.” Do not say that the judges of the former generations were better than those of the present, and therefore I would not listen to them. Instead you are to listen to them, for you are only concerned with them (Maharsha, Rosh Hashanah 25b). It appears that the Sages interpreted in this connection that if one brought first-fruits or a burnt-offering to a priest who was presumed to be qualified and afterwards it was discovered that he was the son of a divorced woman, [a defiled priest not entitled to perform the Divine Service, nor to receive the priestly gifts] the offerings are nevertheless valid, as is mentioned in the last chapter of Tractate Kiddushin. Also included in this principle is [the rule] that a kinsman who has ceased to be a kinsman is qualified, this applying to an elder who may not be judged by a relative, but it is not relevant to first-fruits.

אל הכהן אשר יהיה בימים ההם, even if the priest in the particular generation may not be outstanding in wisdom, you must not therefore treat with less respect than his position warrants. This is why the Torah added the seemingly superfluous words לה' אלוקיך, words usually only used when addressing men of outstanding caliber such as kings and prophets. [If I understand correctly, the author mans that only high ranking people need to be reminded by implication that there is a higher authority than they themselves. Hence the addition of the words לה' אלוקיך after the word הגדתי. Ed.] At any rate, the idea is that seeing that the priest receives the bikkurim on behalf of G’d, it is as if G’d’s agent is addressed by the title reserved for his Master.

הגדתי היום, equivalent to “I have made a public acknowledgment of the great works of the Lord today.” We find similar phrasing in Samuel II 19,7 when Yoav accuses David of having insulted all his ministers and generals by grieving over the death of his rebellious son instead of appreciating the loyalty of his followers who had saved his life under most difficult circumstances. The words there are: כי הגדת היום כי אין לך שרים ועבדים, “(by your excessive grieving) you have made clear that you do not consider that you have any loyal followers.”

כי באתי אל הארץ, that I have come to this land from another country אשר נשבע ה' לאבותינו לתת לנו, as when G’d had said in Genesis 17,8 “I gave this land to you and to your descendants after you.” Seeing that G’d had sworn this oath at the time, I consider myself though apparently an outsider, a stranger, as if I am a permanent resident thanks to His gift, and this is why I have brought these choice fruit that it is incumbent upon anyone who has received the land as either a gift or even as a tenant to present to the real owner of the land in recognition of His largesse.