I have a tankless in my house.
First off if there is a leak from the pipes you will still get water everywhere.
In my case I had to upgrade the gas piping from 1/2" to 3/4" this was due to the size of the unit I installed. This was a pain in the ass.
I also had to relocate the water piping (not nearly so much work) This is because the tank style gets the water in from the top, the tankless gets it from the bottom.
You will probably also have to upgrade the vent system.
Overall I am very happy with the converstion, I would do it again.
Do some serious reading on the installation requirements. Upgrading the gas pipe and the vent might be a deal killer in a basement application.
If there are any other questions, feel free to ask.