TVI uses the same cable as Analog (BNC Coax), and is backwards compatible with analog.
If you replace your analog DVR with a TVI DVR, all your old analog cameras will still work (but still will be analog quality). You can then selectively add TVI cameras in either 720P or 1080P. You not only can keep your old cables, you can even use your old analog cameras as you transition to HD.
If you have an analog DVR and need to add a camera, you can use a TVI camera. It will only record in analog (because that’s all the recorder can do), but when you replace your DVR with a TVI DVR, you will be able to record in HD for the TVI cameras.