I recently switched. It strangely took 3 separate requests (long story) but the third time was the charm. I really don't interact with my Ambassador often, but I have had very promising initial interactions with my new Ambassador. If you are are unhappy, you should ask for a switch, as there doesn't seem like much downside. My only advice would be to be very firm when asking for a switch and follow-up to make sure it actually happens.

As an aside, I have been Ambassador level for 8 years and I can say from experience that I think the focus of the Ambassador program on having a "personal" ambassador is a fatal, structual flaw of the program. It vritually guarantees uneven service delivery (which is confirmed by the many unhappy posters in this thread) and must be an absolute nightmare to manage from a staffing and professional development point of view. I firmly believe the program would be much better off if it transitioned to pools of specialized subject matter expert "Ambassadors". On average, it would be a much better customer experience, it would offer a better professional development path for the Ambassadors and it would be much easier to manage for Marriott. Don't get me wrong, there appear to be plenty of great personal Ambassadors, but the last thing Marriott should want for its best customers is uneven service delivery that creates problems vs. enhances loyalty and the last thing they should want for their best customer service agents is a single tiered career path with no obvious progression and the prospect of being stuck with difficult customers. I imagine there are plenty of Ambassador agents who wish the system was different too!

Anyway, good luck with your switch!