I'm set to check into the Zurich Marriott tomorrow and had booked a Deluxe King Room on marriott.com using a corporate code. I was checking the app this morning to see if I was upgraded (as a Bonvoy Platinum) and saw that the room type had switched from what I booked (it had shown as a Deluxe King Room from when I booked up until this morning) to a Single Queen Room. The amount I'm set to be charged hasn't changed and looking at room availability for the duration of my stay shows my original booking type still available (as well as upgraded types).

I did a quick search but wasn't able to find much, is this normal? Other than calling and requesting my original booking to be honored (or upgraded) do I have much other recourse?

You should prepare for a discussion (either at check in or before leaving the property) by finding and printing a couple hard copies of your original email confirmation, which should show the room type and rate. I'd also do a search for your dates to see what room categories the hotel is currently selling and at what rates or rate differentials. Try to print this information too before it changes or is removed from the website. You should also read Marriott T&C including the Ultimate Reservation Guarantee to see your entitlements for the Marriott brand. You could also contact your corporate travel agent to see whether they can intervene and contact the hotel on your behalf.

It's not normal. Normal is being given the room category that was booked (and for which one is paying) and being told that you've been upgraded.

Has happened to me a couple times. Always at high-demand properties where my corporate rate was significantly below rack. Hmmm...

In all instances raising the issue with the property yielded a quick fix of the �glitch� that they were unable to explain...

Within the Marriott T&C there is a Guaranteed Room Type (Link) which states that "When making a reservation, be sure to note your Member number and room/bed preferences. We’ll always honor your bed type request at our locations throughout the U.S. and Canada. Everywhere else, smoking/non-smoking preferences are honored based on availability; exact bed sizes may vary, and only Titanium Elite members will have guaranteed priority for their requested type of bed and room. If guarantee is not met, guest compensation applies."

Since the OP has reservation that is "guaranteed" but the room type is differing - the OP is entitled to a compensation of $100 USD but from the comment it seems that since the OP is a Platinum instead of Titanium Elite (as per the T&C) - the T&C might not work to the OP's favor. However I would follow MSPeconomist's comments and take hard copies of appropriate booking reservation/details. I would just give a hotel a call or use the chat function within the app to see what is going on but in all honesty be prepared to have a some form of conversation with the hotel front desk/manager.

The one time this happened to me, just the threat of compensation for bed type made my originally booked room magically appear again after the desk said there weren't any available. But you'll have to do this in person at the hotel.

Using the chat my suspicions were confirmed, the room change was in order to move me from the north tower to the main tower. The main tower is nicer for the most part but a less expensive and smaller room is still a downgrade, though they offered to reinstate the room I had originally booked.

Thanks everyone for their input!