Most installed sensors will work with most electronic panels. You should contact a legitimate company, like ADT, and have them give you an estimate of what it would cost to have the system improved to your satisfaction. Even if you don't use them, you can ask questions about what does and does not need to be replaced, and go from there.

Just for the record, ADT will want to install their own PROPRIETARY alarm system and put in a "installer Lock Out" code, so no other company can take the system over.
That locks you into their contract which has an "automatic escalation clause" built in (read ALL the fine print before signing ANYTHING from ADT or it's authorized dealers!)
The escallation clause will "adjust" your monitoring fee upwards every year. I have known some people and companies to be paying in excess of $50-$60 per month for basic monitoring, when other companies do it for $20.
You would be better served by finding a local alarm installer and getting prices.

Steve-
Good points, although it may vary from place-to-place. For instance, I have had ADT for the last 8 years and have never had an increase in my rate. My main point was that the OP can get a lot of useful info by having someone reputable come to the house and provide an estimate. I have nothing against independent installers, but my own experience is that the small, local companies don't always have all the monitoring bells and whistles, and may even simply use an answering service to handle alarm calls.

I disagree with some parts of Kudzu. Most installing companies do *NOT* have any monitoring capability PERIOD. They either are independents that will sell you service from some other company (such as the original poster has) or one of the big monitoring companies like ADT will sell you the monitoring service and contract with a local alarm company to actually install the unit.

While I still pay the local company for my service, they don't handle anything with it. I have separate 24/7 access to the monitoring company (which is nationwide).

As to the original poster, yes I can give you a good recommendation, but by and large nobody will deal with a DIY installer as they wish to protect their deals with their local agents. My best suggestion is to find a local installer that will work with you, explain what they are doing, and give you the access you need to make your own repairs and/or changes.

While there may be some specialized sensors in your system, by and large, they are compatible.

In general, Door/window contacts will work with any system. Motion and glass break sensors may or may not work. Smoke detectors are more proprietary and need to be replaced every 7 years anyway.

Find some local installer to install a system, use something like Alarm.com for monitoring. You may pay a slight premium for a system you can control yourself. The installation manuals are available for most panels online.

Our system was installed and maintained by a local installer before we bought our house and we continued to pay them for monitoring. When it came time to rebuild our kitchen, I told the owner what sensors I wanted, and that I would do the wiring, and that he could install the sensors. He agreed and told me where to leave the wires dangling for him. Later on, when one sensor was not working, I simply ordered another over the web and replaced it, with no issues. He provided me with the installer code as well as other advice over the phone. So I would strongly recommend going with a local company.

A question... With the advent of EVERYONE having a cell phone, what is the advantage of a monitored alarm anymore? You can have the system call your cell and monitor it yourself. What ever insurance savings you get is far less than what you pay for the contract. If an alarm goes off when you are in the house, a burglar is going to be more concerned with trying to shut it up than bothering with you giving you time to call 911. He's going to expect it's been called in anyway. Few crooks are going to hang around with a blaring alarm.

You raise some interesting points that make sense for some people. For me, my monitoring contract costs about $10/month after the insurance credit. In return, I know that I have quick notification for burglary and fire when I'm not home, and that's reassuring when I'm traveling. I also don't like the scenario of trying to find my cell phone in the middle of the night to call the police if there is an intruder in the house. Last of all, my monitoring company does periodic electronic checks to make sure the system is functioning correctly.

Saving $120 per year in exchange for being responsible for my own monitoring is not attractive to me. I'm sure others feel differently. Just my personal 2 cents...

The primary advantage of professional monitoring is for fire, not intrusion.

They will dispatch fire department usually within a minute of receiving the alarm. While you are getting the family out of the house, they will be dispatching.

Intrusion is less useful - in cities like Las Vegas, the police will not dispatch on intrusion alarm unless someone is on premise.

Regarding the monitoring..If you live in a subdivision that has a HOA, they may have a contract with a security service. While you probably cannot have your system call them directly, most (including where I live) will immediately dispatch a security officer if you call them and direct them to. Since they are probably in the area already, their response time is most likely quicker than police who put a low priority from alarm calls.