Using a figure from a PhD thesis, but changed slightly. Copyright?
If you were using the figure only for presentation you could likely use it under "fair use". However, I suspect that you are planning on having this published (I assume yours is a field where conference paper=publication...mine is not), and if that is the case you cannot use the figure or any part of it without proper permissions. A modification of a figure does not revoke copyright. Indeed, it might be a violation in itself. You can't use any of it in your publication.

While I am not familiar with the terms of SpringerTheses, you may find that the person who wrote the dissertation actually retains the rights to the figures. It wouldn't be the first time that a publisher tried to charge for something that they don't actually have the rights to. Why not contact the person who wrote the thesis and ask for permission to use (and modify) the figure? If they still retain rights to it, they can grant you the ability to use it for free.