They have targeted ones they mail out all the time. They also at least 2 times a year offer 2 for 1's during their discount days or 10 days of savings type deals- you just have to wait for the right day. Most but not all only offer the 2 for 1 if you are not a current platinum member and haven't been one for the last either 90 days or so. Many of the deals are targeted for those who would not upgrade without the offer. If you want it bad enough now to pay full price they will take full price. If you could take it or leave it and won't take without the discount (and prove this by not renewing for several months if only available at full price) then a discount will come.
These are some recent Marriott offers I have received. Try them and if they are valid for your account you will see the discount otherwise it will say something like invalid offer. 18000, 18001, or 18003
Did you try calling the Membership department? The code 19000 just worked for me. I actually called in and renewed over the phone, because I had a question for them anyway. CJDidn’t work for me, I’m from Spain so maybe only work if from USA.
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This has been my experience also. Though I can't get them to work on our Marriott corporate account where Platinum expired back in June 2018. They will work on our Vistana account that has never been upgraded.The 19000 and 19001 code does not work on my personal II account that already has platinum-says invalid code
It does work on my Vistana corporate account that never been upgraded to platinum.