So in essence you want to add yet another row (a 'first' one)
to each of your account records, the rest are just there --
the [New Stage] column would become a [Stage] one,
while the [Stage Change Date] to be a [Date] in the result.
This could be done in Tableau in different ways,
I've been using a UNION of the datasource table for that.
Then simple row-level calculations would do the rest.
Please find the attached.
Stages_v10.2_YF.twbx 18.7 KB