First, the SDA uses the methods of deception in order to convince people to "keep the Sabbath."
a) No one "changed the day" in the sense the SDA claims it was changed. In the Protestant Reformation, the debate with the Catholics was over "Sola Scriptura" (Scripture only) as one's guide to Christian living and salvation. The Catholics CLAIMED the Protestants were following their lead when it came to Sunday "worship" by claiming they "changed the day" from the Sabbath to Sunday. This was false, and the Protestants called their bluff on this in the Augsburg Confession where they showed, using the Catholic's own historical documents that preceded the Catholic church and why Christians began gathering together on the first day of the week, not for "rest" as the Sabbath commanded, but as a day of corporate worship and prayer. Rest is not worship, and worship is not rest.
b) The Sabbath was based on a lunar calendar, where the week was reset every new moon. Now we use a fixed week calendar. So the REAL Sabbath wanders through our week. Even the leadership of the SDA knew about this, but recognized this information would not go over well with their members, so they perpetuate the lie that the Sabbath is Saturday.
c) The wording of your question makes me suspect you are in fact a member of the SDA, and you are just trying to be clever here. Real Christians do not "worship and honor a day" whether it be Sunday or the fake Saturday Sabbath. We worship and honor God, according to Christian Liberty, where we are free to do this on any day we so desire, and at any location we desire. Specific days are, in the end, irrelevant. The early Christians saw a significance regarding the first day of the week, and began this practice, either gathering together very early in the morning BEFORE GOING OFF TO WORK or later in the evening AFTER WORK like what we see in Acts 20 which, by the way, is the ONLY example of an entirely Christian gathering, and it was on the first day of the week.
d) You touch on being now under grace. Should you embrace law, Scripture says you fall from grace. Should you embrace their truncated law, you will also be embracing a false gospel, for they do indeed teach that you have to "keep" the Sabbath in order to be saved, through the back door, as it were, claiming you lose salvation should you fail to keep the Sabbath once you "understand" you should. It is quite convoluted.
e) You misquote Scripture, out of context:
God said, if you want to enter life, obey the Commands! And wouldn't that include keeping His Sabbath?
Rule number one when it comes to biblical interpretation and understanding: Who was speaking, and who was being spoken to? Jesus was talking to a Jewish man, who was under the law, who asked what he, personally, could do in order to save himself. Jesus said if he could keep the commandments, he could save himself. What comes out in the narrative is that he did not, and could not keep them, with the conclusion of the matter being the disciples being confused, asking, "who then can be saved?" For they understood that no one ever kept the commandments, given the spiritual implications Jesus brought out. Jesus concluded the discussion by pointing out that with God, it was possible. How? Because we are to be saved as a matter of faith, and not the law, or the works of the law, whereby NO ONE will be justified. So here's the SDA trying desperately to justify themselves before God in order to be saved.
If you are truly looking for the answers regarding these things, do an internet search for "Lying for God" which is a pdf book you can download from a number of places, that exposes the fraudulent activities of the SDA and their theology.