Name change during the course of PhD?
I would recommend sticking with what you started using, since there is no real advantage not to. That said, if you really want to change I don't think a change in first name will affect you much. Depending on the citation style, the name change won't even be visible. I don't think this particular change is anywhere near a change of last name in terms of impact on your popularity.

With regards to Google Scholar: you can always add your new publications manually to your profile in the unlikely case that scholar borks, so that's not an issue.