Just to play devil's advocate, I'll note that the Marriott Rewards Terms and Conditions also says (emphasis mine):

The Company reserves the right to reject applications for Membership, to revoke, cancel or suspend any Rewards Program Membership, elite membership status (including Lifetime Silver, Lifetime Gold and Lifetime Platinum status), Reward, and/or any and all unredeemed Points or Miles, or take other action at its discretion, at any time with immediate effect and without written notice or liability to any Member, if Company believes: (a) the Member has (5) engaged in abusive, fraudulent, inappropriate, or hostile conduct in connection with the Rewards Program, the participating hotels or their guests or employees, or the Company or its employees."
Link: https://www.marriott.com/rewards/terms/default.mi

It seems clear that the only reason you made the modification was to get out of the cancelation fee to which you agreed. If the hotel figures that out and can prove it to Marriott Corporate, I see a case for you still getting charged. Of course, all of that happening would require someone at the hotel to play amateur detective and figure out that's what you are doing. That would be a remarkably foolish use of labor just to potentially get one night worth of revenue. (Although, posting your intentions on a public website probably doesn't help.)

The likelihood of any of that happening is slim. And for the record, if I were in your shoes, I'd probably do the exact same thing.