Planner is not a replacement for Microsoft Project. You can’t directly import an .mpp file into Planner. Microsoft never built that bridge. Planner is deliberately lightweight; Project is not. Trying to force one into the other is like pouring concrete through a funnel.
> If you care about dates, dependencies, critical path, or resource load → stay in Project
> If management insists on Planner → flatten the plan and accept the loss
Trying to “import MPP into Planner” is a business decision problem, not a technical one.
Moving to Planner means
> You accept Planner’s limitations
> You’re willing to maintain the flow
> You don’t expect “full project management” at the other end.
That said, why are you trying to move from mpp to Planner anyway _ Microsoft Project Desktop Client ISN'T going away - only Project Online. If you need a replacement for Project Online then look at Altus.Pro which offers direct integration with Project Online and capabilities beyond the current ones in Project Online amyway.