[Sprint Samsung Galaxy S 4 General] MasterSe7en:
This is the method i used to downgrade the modem, NOT the bootloader, i couldnt downgrade the bootloader. My main problem that i was having was not switching from LTE/CDMA to WCDMA/GSM but ya i only followed that guide up until the modem downgrade, after that is a lost cause because it didnt work until i used your method

http://galaxys4root.com/galaxy-s4-tutorials/how-to-unlock-sim-on-sprint-galaxy-s4/

---------- Post added at 03:06 AM ---------- Previous post was at 02:54 AM ----------

ok let me explain the differences lol...its kinda....confusing at times if you dont understand...

If you unlock a Sprint phone, you can use it for Sprint/Tmobile/AT&T and that depends as well with what features will work.

If you unlock a Tmobile/AT&T Phone, you can only use it with AT&T and T-Mobile(or other GSM Variants)

A newer Verizon phone comes pre-unlocked...meaning you can just toss whatever sim card you want in it and set the Network mode to Global, and it will work, again with limited features i.e. you may not get better than EDGE Data on some providers, or you most likely will not be able to get LTE to work on anything other than the network it was made for, however newer phones like my note 3, after i unlocked it, also get LTE on AT&T, but that depends on the phone.

you Cannot use a Verizon phone, unlocked or not, on Sprints network and Vice versa.

Sprint and Verizon are what you call CDMA Carriers, technically if it wasnt for LTE, Verizon and Sprint would NOT have sim cards, if you notice, all their older phones do not have sim cards.. the sim for Verizon and Sprint came later when they aquired LTE. a Verizon CDMA radio is usually locked to their network, unless it is FLASHED, then it MIGHT be able to be modified for a different network, but it usually comes down to hardware, if the hardware doesnt exist, then it cant be modified. Same for sprint.

However since sprint and verizon now use GSM frequencies to enable the use of LTE, a sim card is needed, so the GSM radios can usually be unlocked or manipulated in those phones....but if you unlock a Sprint SPH-L720 you CANNOT use your verizon Sim in it, just like you CANNOT take a Verizon sim out of an Iphone and put it into an AT&T Unlocked iphone...it just wont work, theres no CDMA Radio (this info is subject to change because im almost sure that all 5s's have both CDMA and GSM in them regardless of carrier).

BUT you CAN take a AT&T Sim card out of a phone and put it into a Verizon Iphone 5 or better and it will work without ANY modification other than APN settings :)

So to sum it all up.

CDMA-to-GSM(Verizon/Sprint) can be unlocked to use AT&T/T-Mobile BECAUSE they have GSM Radios in them for LTE.

GSM-to-GSM(Tmobile/AT&T) Can be unlocked to use T-/Mobile<>AT&T & AT&T<>T-Mobile, but CANNOT be used for Sprint/Verizon.

GSM-to-CDMA---No.

CDMA1-to-CDMA2---No.

(Verizon)-to-(Sprint)^

but if its something like Boost mobile...Boost uses Sprints network....

Sprint-to-Boost--Yes but needs to be flashed via CDMAtool or somekind of direct modem flashing utility like a Z3X box

i hope this helps some of you guys understand...again its Effing confusing =/