Considerations
No two setups are the same. Your plan of action will be determined by a set of conditions. There are a number of questions you need to assess.
Does your existing DVR support IP cameras? Which IP camera manufacturers will your existing DVR support? If not, what are the options for an NVR? Should you use IP video management software?
One difference between images transmitted in a digital format and those sent as analog signals is that the former do not depreciate over distance. So, using an encoder to convert analog footage to digital will ensure that the quality of images is preserved when a camera is placed a long way from the recorder. You may want to assess the impact of long-distance analog cabling.
It is not always the case, of course, that analog cameras are irreplaceable. They may be at their end of their useful life if, for example, you decide that higher-resolution cameras would serve you better and justify the extra investment. Bear in mind, though, that megapixel cameras will probably end up raising the storage burden of your system, as high-intensity cameras generate bigger files.
Digitalizing analog footage will also affect your customer's bandwidth; you may want to assess the impact that has on their network.
You will want a single video management system to record and access all cameras. IP video management software only supports IP video streams, so you will need to purchase encoders to convert the analog video stream from your cameras into an IP one that the IP video management software can handle.
Ultimately, encoders bring many of the benefits of IP cameras without the need to install and recable them. They create a similarly IP-enabled network at a fraction of the cost and in a fraction of the time. The intelligence that these boxes bring to each channel makes video access fast and flexible and improves the quality of pictures. They are cheap to manage in the short term and help make savings in the long term by futureproofing technology. Now, that is a nice picture.