I have a solution for this issue.

1. No content will be gated. The whole of a raid instance will be available immediately on release of the patch.
2. The content will be tuned to require elite play on the order of Ensidia, et al. But the content will also have clearly defined and pre-built "step-downs" that will allow the instance to be nerfed in a predetermined way. (Conversely, the content could be tuned for regular raiders, but have pre-built step-ups that would be active when the instance is released.)
3. The high-end tuning will be maintained for 3 weeks (or 4, or 2, whatever is deemed appropriate) to allow high-performing raiders to make their attempts at clearing. The loot tables will not differ from lower-end tuning. During this period, however, achievements for downing bosses or accomplishing encounter objectives will have an "Elite" tag to clearly differentiate them from "normal"-tuned achievements.
4. After the determined period, the instance will be nerfed according to the plan, and the Elite achievements will no longer be available.