I agree with Ignacio, do not use 12V connector with 19V because 19 is more than 12.
At such low voltage probably nothing will happen, but you don't know why it is rated for 12V. Maybe insulation can handle much more, but if you overrate it by more than 50% - you may expect some undesireable effects (including fire, corrosion, changing insulation mechanical or electrical properties).
I would suggest to use Lenovo socket and plug from T43/T60/T61 series laptop. It is well designed and hard to break. Best power socked I ever saw in any laptop. I never had problems with power connector in any ThinkPad (I was stomping on cord many times, pulling it accidently etc.). It looks like this:
Notice that this connector is not supposed to be soldered to board. In my laptop - this connector is surrounded by some rubber and it moves with plug. It is for motherboard protection. In low cost laptops power connectors are soldered directly to board and if you pull cord accidently - you can break whole motherboard.
For plug - just replace whole cable in your power supply. There are cheap cables like this on ebay:
image source: ebay auction link