With the current design of setFCV, on downgrade the FCV document / server parameter looks like:

{version: "8.0",
 targetVersion: "8.0",
 previousVersion: "9.0"}

And on upgrade it looks like:

{version: "8.0",
 targetVersion: "9.0"}

Thus we have various places in the code that assume previousVersion can only used for downgrade, for example here and here.

For Symmetric FCV we will add a new state to the FCV document:

{version: "9.0",
 targetVersion: "9.0",
 previousVersion: "8.0"}

 
So we should remove that assumption.