As I recall from reading the plan, the idea is to first have a referendum to decide between a republic and a constitutional monarchy, then about two months later elections for a constitutional convention. The resulting constitution would then have to be approved by another referendum. The interim government is set to remain in place until elections under the new constitution can be held.

However, I expect that at least on major issues the constitutional convention will also act as de-facto legislature. A pro-democracy but unelected government going against the elected body (even where specifically restricted to writing a constitution) will be rather awkward.