Least Destructive New Heating System? Hot Water or Air?
I would maintain a hot water heating system for the increased comfort. Forced air is actually quite uncomfortable to people but it is cheap to install.

Just google "ideal heating curve" and you will see the many graphs comparing different types of heating systems. Like here. And here.

So you can choose, save money now and suffer or spend more now and be comfortable.

Good luck with your project!