Question about redeeming Marriott Rewards points for a hotel stay

I'm thinking about redeeming my few Marriott Rewards points towards a hotel stay and buying points to cover the rest of the stay. If I buy e-certificates and attach them to this reservation, what happens if I need to cancel? It's a little unclear in the terms and conditions.

Would those e-certs be returned to my account so that I could use them elsewhere or do I lose them? There's a slight chance my plans might change, so I don't want to spend $350 on e-certificates, need to change/cancel my hotel for those nights, and then lose my money.

I'd really appreciate clarification from any of you that might have done this in the past. I don't travel all that often, so I need help from the travel vets!

Thanks in advance!