how to downgrade i9300 from 4.3 t0 4.1.2
I needed to "repair" imei on a galaxy s3 with factory 4.3 software .
without reading up, I know now that the modem is somehow tied in with the baseband and you get reception errors afterwards.
sooo... after searching 100's of sites I finally managed it.
so this is exactly what I did, im not saying its right but its worked for me !
1) downloaded "I9300XXELLA_I9300OXFELL1_XEH VER 4.1.2" from nspro support site and flashed it to my s3.
2) used nspro to "repair" my imei number.
3) phone rebooted but signal dropped in and out.
4) used odin to flash "modem_RIL_CWM_I9300XXELLA" file to phone from [REF] Galaxy S3 RIL+Modem CWM Flashable Collection - xda-developers
5) flashed "I9300_IMEI_Baseband_Repair" from nspro support area to phone.
6) phone rebooted with yellow writing test screen as expected !
7) repaired imei again using nspro.
8) realised that step 2 was a complete waste of time as I had to do it after step 6 anyway hahaha
9) held volume up/power and home to enter recovery screen and did full factory reset and also reset cache
10 rebooted and phone has full signal and keeps it !
so that's my method, thought it might help someone, please feel free to comment