You can apparently lose your Tier-rank soon.

[Araxes]

De-ranking will only affect Crystal and Diamond. For 6.2 you won't fall below Platinum.

Thanks for clarification.

Pretty sure you get rewards based on the highest tier you achieved, so if you hit crystal once and derank to Plat 5, you'll still receive the crystal rewards.

Well then why the demoting in the first place? It makes no sense. Ah well, SE does make many desicions that make no sense^^