Bottom line - if it improved things with the CM19A (and is isolated from mains voltage) then leave it. Using 50-ohm cable/connectors would have been best but it's not supercritical, merely less than optimal. That's assuming you only changed the receiver antenna. Changes to the transmitter antenna are against FCC regulations.
However, I would suggest doing it right if you plan modifications to the CM15A antenna. If you read the Wikipedia page Brian referenced, each impedance mismatch causes reflections which are undesirable. And, the prohibition on changes to the transmitter antenna applies here, as well.
I'm not familiar with the CM19A but have used the eggbeater antenna with many other receivers (e.g. RM1SG) and the results were always 100+ feet for reception with a single, centrally located antenna, increasing to 300+ feet with a pre-amp. Even building it with insulated copper electrical wire (e.g. 14AWG) and using 75-ohm coax for the phasing line (and only for the phasing line) will give good results.
If the CM19A uses the same receiver as the CM15A and the recent TM751, the frequency mismatch I mentioned earlier is likely to be a major part of reception problems. A better antenna will only bring marginal improvements in this case.
Finally, folks (and especially noobies) need to be extremely leery of antenna modifications to any device that plugs into the wall. Some, like the CM15A, isolate the electronics from mains voltage but others, like the TM751 & RR501, do not and adding an external antenna without isolating it can be fatal. If in doubt, don't touch it. There are schematics available for the CM15A, TM751 & RR501 so it's easy to determine which are isolated but I haven't seen schematics for the CM19A nor for security consoles so I never recommend antenna improvements for them. If I had any confidence that X-10 will continue to supply them I'd buy a CM19A and test it but I'm unconvinced by X10SupplyDepot's assurances otherwise given the number of already discontinued devices.