This is of course a variation of the grandfather paradox. here is an interesting video about it.
My take on this is this: #No. Even when omitting the obvious paradoxes and creating the situation where you replace your grandmother, you are not just the sum of your grandparents genes. You are also the result of (however minute) random mutation. You could obviously go back and get pregnant from your grandfather, but the grandchild you produce would not be you. Therefore you and your grandchild you could not exist on the same time line.