Not only is changing laws harder. Changing regulations requires following the Administrative Procedure Act. They might also be short circuiting APA - as in typical for this admin to attempt.
What part of this memo changes the regulations? The punchline is this: “Where adjustment of status is in the discretion of USCIS, officers are reminded that they are to consider all relevant factors and information in the totality of the circumstances in exercising that
discretion.”
All the memo is saying is reminding USCIS officers that adjustment of status is an act of administrative grace and applicants aren’t entitled to have their status adjusted. That’s always been true.
That is what the memo is trying to say. That no regulations or laws are changing, but it's simply pushing for a certain interpretation of the law that doesn't seem to have been the status quo for awhile.
Some stats I found online report that ~60% of greencards are granted to people already living the US. This memo makes it seem like that route will now be much harder. So while its true that the law as written has always been true, they're definitely pushing for a change that will result in past behaviors no longer being true.
Except it is completely contrary to how the immigration system has worked in the US for decades. It is absolutely standard for people who are already in the US on other types of visas to apply for Green Cards.
All the memo is saying is reminding USCIS officers that adjustment of status is an act of administrative grace and applicants aren’t entitled to have their status adjusted. That’s always been true.