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.