Reading the situation
Turbo HD, other analog standards and older coax installations can coexist. A physically compatible connector does not prove that the camera, channel standard, power supply and recorder function work together.
A DVR works with a different infrastructure than an IP-NVR. Therefore, it is important to first determine which signal, which cable and which recorder are actually present when expanding or experiencing a malfunction.

Turbo HD, other analog standards and older coax installations can coexist. A physically compatible connector does not prove that the camera, channel standard, power supply and recorder function work together.
Read the type of recorder, channel description, and camera model. Follow the cable routing, check for separate power supplies, and look in the recorder to see which standards and resolutions are supported per channel. Also note which recording or playback function should be retained.
Choose retention, targeted replacement, or migration to IP based on the required features and the state of the infrastructure. Test one camera on a free or agreed channel first before changing multiple existing connections.
Check live view, date and time, recording, playback and night view of the test channel. Record the chosen standard and channel name, so that a subsequent change is not based on assumptions about the cabling again.
No. The supported standard, resolution and channel capacity vary per recorder.
That depends on existing cabling, desired features and expansion plan. Compare the full installation, not just the recorder price.