- You should do what you are funded for simply because you signed a contract to so. End of discussion. But the point is: It is actually not in the best interest of the funding agency to hold you back in case you found a more exciting topic along the way. Are you obliged to comply with the funding agency against its own interest?
- Your funding agency may not be interested in general research excellence but may have more narrow interests. But even in this case there could be ways to satisfy those more narrow interests outside of your original proposal. Should you grab that opportunity?
- Your original proposal was certified by peer-review to be productive and worthwhile, switching to a different topic is too risky. But I don't think this makes sense either: It puts the opinion of your "peers" above yours about your own very special research topic.
Is a research proposal a "lower bound" to your research and should you modify parts of it as you go along? Or is it cast in stone? Any opinions, experience?