First of all, don't panic. Second of all, there is nothing to be panicking about..... its a very easy fix and you don't need to use the Ariza patch for it or any other for it.
I tested this on a Galaxy S3 I9300. But I am 90% sure that it can be applied to other models as well. But as they say, you do everything on your own risk, I am not to be blamed for any thing.... what I did fixed my cell phone. Just make sure to keep a back-up even of this scenario, keep a CWM back just in case. BTW this procedure cannot do much harm any ways since its just flashing a new rom but still you are on your own
I was involved in this situation just a couple of hours back, was trying to find out how to get my mobile to work properly. I actually flashed a stock 4.1.2 firmware (KLC/KL4/XEO) over a working paranoid android 3.99, every thing was normal after the flash except that i was not registered on the network. I couldn't make any calls or send any texts..... I wanted to just kill my self for not taking any back ups, imagine life without a cell phone....
Well to cut it short, in my 2 days of research i found out that if you install an old firmware over a newer one.... NOTHING CHANGES... its just that the old modem cannot read the NV information on your cell phone. Your phone is not bricked, neither it has any issues nor you need to get it to a shop.... and your IMEI hasnt changed as well. What you need to do is:
Just update your modem (download an atleast MG4 baseband firmware), I tested with the 4.1.2 stock MG4 firmware BTU (United Kingdom) downloaded from sammobile.
After the download finishes unzip the file and then remove .md5 from the full file name of the extracted file, you will be left with just the tar. Untar it and inside you will see a modem.bin file.
Use odin (windows) or heimdall (mac os) to flash just the modem.bin file and that will be all.
PS: For odin select PHONE and for heimdall select RADIO. I am positive that it will work flawlessly, just make sure you have your efs folder there... it doesnt have to be backed up it just have to be there.