The numbers in the book often don't match that table in the DMG, and are frequently lower.
Oh, there are some issues IME, but most of them are minor and they have been correcting them as they go. Most monsters in the MM are very close to the CR calculation you get from the DMG (I've check a bunch of them over the years). If that is the thing you are talking about, again, they have already been making those type of small changes in the newer monster book "previews."
I thought you were talking about the math in the DMG.
There is also the bigger issue of encounter building and stuff per day, which almost certainly will be touched on. But there is also MM math.
I also thought you might be talking about encounter design, which is completely different from monster math. There are clearly making some small changes to individual monsters, but not wholesale changes to monster math. That isn't needed IMO. The math works, just a few monsters need to be tweaked a bit.
Preview? You mean in some recent book(s)?
Yes, the specifically said the revisions in MotM previewed the plans for monsters in the 2024 MM. But those revisions were very minor and sometimes nothing changed. They typically didn't change the math, but how they operated (like removing spell slots and making all casters innate casters). A few monsters got a couple more or less hit points and some did more or less damage, but not a lot and not by much.
And they still have to deal with MM monsters.
Yes, they will update the MM monsters, they have said that. My point is simply that the monster math will not change, just a few monsters will have revisions to be in line with the existing math better. Most of the changes will be to spellcasting and, hopefully as was hinted at, to add more interesting actions and traits. More in line with post MM monsters (which use the same math as MM monsters).
They have said monsters will be in the playtest. And they will want people to buy the new MM--or the D&D beyond equivalent, so there has got to be changes.
There will be changes as I have noted above, but less with regard to the math IMO.
However, I want to stress this, if people want to buy a MM at the end of 2024, they will have to buy the '24 MM. There will no longer be a '14 MM. This is more of a reprinting with a errata than a new book. They do not need to make changes so much as replace one with the other. The MM is still selling well. A little bump from people repurchasing is nice, but the focus is maintaining the flow IMO. Just sell to new buyers.