How do you unflash an SCH-1545?

Several years ago when flashing was necessary, I flashed my 4G Samsung Galaxy to 3G in order to work with 3rd party providers. I've recently been having problems with it and have been told I need to "unflash" the phone. Several technicians have been VERY clear that this is not as simple as a factory reset and unless I find someone who knows exactly how to do this, I could easily render my phone permanently unusable. Is there anyone out there who was well versed in flashing phones that can please guide me on how to unflash my phone? As much as I'd really prefer it, I simply can't afford to buy a new one.

In case any of this matters:

Model: SCH-1545
Android version: 4.4.2
Hardware version: 1545.06
Security software version: MDF v1.0 Release 2

I can get you any other information you need from the phone (I might need a little guidance on some)

Thank you in advance for anyone who can help me!
Allie

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