Is there any way to change victory conditions of a game already in progress?
There is no way to do that with the base game.
You can do that by using a custom FireTuner panel, as detailed in this thread on civfanatics.com. There's a download link for the panel there, as well as information on how to add this panel to FireTuner and how to use it.
There's also some useful information on that thread for those new to FireTuner:
Be aware that changing victory conditions mid-game might seriously screw with the AI.
Oak's answer is probably the easiest way to do this, but it has two problems:
An alternative approach is to edit the save file: