This is accurate. Once you've received the air miles portion of the travel package, any changes to the hotel certificate turns the cert into a "partial." This designation should not impact your ability to book in any way (and if a rep says it does, HUCA. They don't know what they're talking about). Your "partial" cert is still good for 5 or 7 nights at the category level that it applies to.

Also, the point value assigned to the cert (ex. 135,000 for a partial OC8 or 105,000 for a NC5) just means that if YOU elect to cancel the certificate, that is the value you would be refunded in points (again, because they've already applied the remaining points toward the airline miles portion). That's the primary reason for calling it a "partial" cert--it's a way to let the Marriott reps know that they should not refund you the full purchase amount because you already received the airline miles portion of the package.