Well according to reddit, you can check Here to see if an offer exists to upgrade your existing Marriott Card to the plus, You can take the survey here to report here on what offer you get (thanks u/kerrigjl).
Above you see my three year card holding brother had gotten an offer of 20K points to upgrade right to the plus. Even though he could close the card and reapply for the plus for the 100k points offer. According to the New Plus Card terms and conditions:
The product is not available to either (i) current cardmembers of the Marriott Rewards Premier or Marriott Rewards Premier Plus credit card, or (ii) previous cardmembers of the Marriott Rewards Premier or Marriott Rewards Premier Plus credit card who received a new cardmember bonus within the last 24 months. If you are an existing Marriott Rewards Premier customer and would like this product, please call the number on the back of your card to see if you are eligible for a product change. You will not receive the new cardmember bonus if you change products.
What is unusual is my wife did not get an offer even after 3 years of card membership, where as my mom on year 2 has a 20k offer. The offer also shows the differences between cards:
For 5/24 sluts such as myself, I could never get either card. I instead downgraded my Ritz to a Marriott and got this offer today:
My advice is those that could never, ever get under 5/24, is to go for it! You would never be able to get the 100k offer with a new plus. For those ready to clean up their act, then close the card and get a new Marriott Plus when 5/24 clears.
For those those get no offer, are under 5/24, and have gone past 24 months for the first bonus, then it might be worth moving credit off this card and closing the account to apply for the new card.
Now the ultimate AD Churn would be get the Marriott Plus for 100k bonus when under 5/24. Then 4 ore resticted cards (Ink, CSR, Freedom< United Club). Then the Ritz under 5/24 exemption, then convert the Ritz to another Plus to maintain he MLA waiver on top of getting another Ritz like I did for my previous back door strategy outlined here. Of course I haven’t hit the initial bonus on my MLA Ritz yet, but I am willing to host any successful datapoints!
Post those Marriott-Plus referals if you got them!