Does anyone know if after you turn on SuitePromotions you can turn it off and go back to Advanced Promotions?

I strongly suggest migrating to SuitePromotions and staying on it. It is far superior.

That's the plan we just needed to be sure whilst we run both the Elbrus site and the new 19.2 site at the same time we don't run into any unexpected issues once we enable SuitePromo as it isn't currently enabled in the production account.