Marriott is usually pretty good about giving notice of major program changes. So in general, I don't think you will wake up one morning and find that the United TP awards have disappeared with no prior notice being given.

I can't say they have never made "changes" without first giving notice, but those are usually smaller changes (i.e. individual hotel category changes, etc.) The times they have added or taken away whole reward options they have given 3-6 months of notice.