Your obligation as a taxpayer is to report your basis properly such that the gain or loss is correctly reported. 

EMC to DVMT to Dell was a series of complex transactions - accounted for as "sales" - that took quite a bit of effort on shareholders' part to report properly.  Throw in the additional complexity of acquiring your original stock via an ESPP, the sale of which can create "compensation" (which adjusts your basis) and if you've got that all straight then I tip my hat to you.

If you know that the basis reported by the broker is wrong, and can defend that position, then you certainly can change it.

Tom Young