The Daijōsai is an important Shinto ceremony that is conducted by each Tennō at the beginning of their reign. I wrote an essay about it around the time of the current Tennō’s accession, which you can buy on Amazon (affiliate link!). The history of the ceremony is not continuous, because there were a couple of centuries in which the Tennō did not have the resources necessary to perform it — it requires several buildings to be constructed from scratch, and then demolished, among other things. However, the Tennō always wanted to perform it, and once Japan emerged from its period of civil war, they worked on the Tokugawa shoguns to get permission and funding, and ultimately succeeded.
The written records suggest that the ceremony began in the mid to late seventh century, but the early records do not contain much detail. From the tenth century onwards we have more surviving detail, and we know that a great deal was preserved over the centuries, and then recreated at the restoration. There are some elements that have been lost or changed — inevitably — but there does seem to have been a consistent intention, largely achieved, to maintain the early form.
So, an important question is “how early?”. This is where archaeology, and a feature of Japanese history, help. In the eighth century, very soon after the ceremony started, the Japanese capital was in Nara, but it moved at the end of that century, and after a couple of false starts landed in Kyoto (which means “Capital City”) for a thousand years. Nara did not disappear, because it was home to several important jinja and Buddhist temples, but it did become a bit of a backwater. This means that a lot of it has not been thoroughly built over in the last 1200 years, unlike Kyoto, and thus archaeological sites remain.
The sites of the ceremony itself have been partially excavated in the past, and revealed that the layout of the temporary buildings was fundamentally the same as the current design, albeit somewhat less elaborate, from the early eighth century. However, that does not tell us about what was done in the ceremony.
A discovery in Nara last February does, and an article in the January 1st issue of Jinja Shinpō talks about it in some detail. A substantial collection of wooden labels was found near the entrance to the old palace. These wooden labels (called “mokkan”) are a very important source for Nara-period history, because they often survive with writing on. The ones found this time are for the Daijōsai of Shōmu Tennō, which was held in 724. They seem to have been for the food and materials gathered for the ceremony, and the food offerings largely match those recorded in the Engishiki, a tenth-century text. There are also labels for two “mats for the kami”, which strongly suggests that the mats that served as seats for the kami (Amaterasu Ōmikami in this case) were already in use at this early date.
Obviously, this does not tell us that the ceremony was conducted in exactly the same way in the eighth century as in the tenth, but it does reduce the reasons we have for thinking that there might have been substantial changes. We know that the ritual buildings and the ritual materials had a lot of points in common with those used later, and there is no evidence — as far as I know — for striking points of difference. The evidence is, therefore, good enough to shift the burden of proof.
It is even, I think, good enough to shift the burden of proof all the way back. The archaeological findings are not from the earliest Daijōsai, which were not conducted in Nara, but they are from within 40 years or so of the start of the ceremony. There was one major change, which we can see in the written records (originally, it was performed every year, but in the late seventh century it was scaled back to once per reign), but no sign of any other alterations. Thus, it is fair to say that, if you want to argue that the Daijōsai performed in the 21st century was fundamentally different from that performed in the seventh, you have to provide some evidence. There are very few ceremonies anywhere in the world for which that is the case.