I do not know if you can downgrade to a Zync, but I do know that Amex does not allow you to upgrade a Zync to any other charge card, and I suspect that the same applies in reverse so I doubt you can downgrade.
If you want to do a new application just for a cheap MR card to keep your points alive, I would suggest the Blue. As stated by the poster above, the points will be kept alive, but won't transferable until you get another full fledged MR card.
Why I would suggest a new application for the blue, Amex Zync is lumped together with all the charge cards when it comes to Sign up bonuses. So if you look at the new application for Blue (you may need to clear your cookies or else you may not see any offer) they will offer 10K points to new Blue applicants. Zync will offer 10K, but will not grant it if you had any prior charge card (Green, Gold, Plat) within the last 90 days or the Zync within a year.
So for a fresh application, with the blue you'll in the very least get a bonus 10K, if you did decide to go with Zync, the limitation I stated above also counts the other way around, so if you wanted to apply for say a gold card later, by the current rules you'll be ineligible for the sign up bonuses because you have a Zync, unless you canceled it within 90 days of application, which would cause you to lose points anyway...... so yea, keep it simple and go with blue.