You have a few issue to consider. In the first place, most of the sensors (and systems) being installed today are wireless. Easier to troubleshoot, to move, and to add (as in your current situation). Hard-wired is becoming a thing of the past: it still works, but we are evolving away from it. In the second place, your old system probably uses your phone line to communicate - also old news, as burglars cut phone lines, and what if you want to go all cellular? You are stuck. The safest and best systems today are cellular for that reason - and do a ton more, like remote control and emailing you with events. Then there is the possibility that the old company programmed a lock-out code (common practice) making it hard or impossible for another company to access your system. And finally, you can get remarkably good system these days that are quite inexpensive - as in $99 for a base system that you can ad to any time, and it includes the cellular radio for calling the monitoring center. The world has changed for alarm systems.

You might want to search on line for a system, as an alternative to upgrading your system. Thee are several options - and make sure you check the reviews of any company you consider, including the BBB.

Your existing wired or wireless sensors, wireing and siren can be reused. It is inexpensive to replace the alarm panel ($100 or so). If you have existing wired sensors, you should reuse them and not change to wireless. Wireless is used because it saves on the cost of installation. In this case, it sounds like most of your sensors are already in place. You may want to consider wireless if the wireing can't be run conveniently. However, the cost of a wireless contact is $30-40 compared with $3-$5 for a wired contact. You also will have to replace batteries eventually on the wireless transmitters. With wired contacts you won't have to worry about this. Most systems will easily allow a mix of wired and wireless contacts. If you want to DIY, you can by the same parts online that the pros use. Internet monitoring services such as nextalarm.com or alarmrelay.com will allow you to monitor for 30% of the cost of a traditional alarm company.

Since I sent this message I did some additional research. the current hardwired system is Brinks equipment, Yes the installed kept all the "codes" there is apparently no way to cost effectively get into the system. It already is hardwired for 5 doors, 2 CO2, 2 motions and about 20 windows. Although a complete amateur, I am a committed DIY'er and have done a fair amount of electrical and computer wiring, so I am game for trying to save as much of the current system as possible and installing a new panel, and keypads myself.
I think I would like to install a hybrid wired/wireless because I'd like to include my detached garage in the system. I am also intrigued by cellular connection and email alerts. The big names seem to be Adamco, GE, DCS...is there anyone else I should be considering?
Is any of these brands inherently easier for a DIY'er to install and maintain? Or are there other recommendations?

Thanks!!!

I am only familiar with Ademco. While their stuff isn't simple, I was able to figure it out by reading the manuals which I downloaded online. The panel that I used for my system is a Vista 20p. There is a newer (and slightly more expensive) version called a vista 21IP. The Vista 21IP also allows alarm reporting via the Internet, not just via phone line. Vista 21 also allows an on-board add-on GSM cellular module. Vista 20 can be expanded for these capabilities via an external box (7845i-GSM). The Vista 20 and 21 allow up to 16 zones out of the box. You can also add zone expanders (4219 or 4229) where each one adds an additional 8 zones. You can group some sensors such as doors and windows by room, i.e. a single zone for all windows in a room. Doors you can also group, if you want. You probably want to keep your CO, and motions each on separate zones. You probably want to add one or more smoke detectors for living spaces and heat detectors for attic and garage. I would suggest that you consider alpha keypads. Alpha keypads allow you to see the fault zone in plain English (bedroom window fault) rather than just "zone 5 fault" where you have to look up or remember what zone 5 is. A single alpha keypad is also required for programming of the Vista 20 series. Since you want to do some wireless zones, don't forget to add the wireless receiver module and wireless contacts.

A great resource for questions is the diy forum of the home security store. Most of the on-line equipment sales places will answer questions as well.

Here is a link that might be useful: DIY Security Forum

The Brink's equipment was designed to be impossible to "take over" by another company - one of the only things people did not like about them as a service provider. The various wireless components are designed to work only with their respective alarm panels: GE sensors with GE panels, Honeywell sensors with Honeywell panels, and so forth. As for wireless, GE are the easiest. Some companies sell a basic system at a deep discount, and additional sensors at close to wholesale - so it could in fact be cheaper to start over with wireless, and get cellular monitoring built in, along with interactive features. The best of the cellular/interactive providers is Alarm.com, and their services are resold by a select group of dealers across the US. Look at reviews, it's the best way to find the good guys.