Flashing Custom Roms on EVO 3D Flashed to Ntelos Network

[jesimpki]

Hi all, I'm new to the ROM flashing scene. I'm currently awaiting delivery of an EVO 3D. When I get it, I will have it flashed from Sprint's network to Ntelos. I called the local authorized retailer and they told me they can do it without a problem for one time $50 (I have no idea how to flash a phone myself and don't want to screw it up). I asked them since they mentioned that no future OTA updates would be possible if flashing custom ROMS would be an issue (like CyanogenMod) and they advised against it due to the possibility of accidentally flashing it back to Sprint. Can anyone tell me if I can safely root and flash my EVO 3D to CyanogenMod or will I run into what the retailer warned me about?

Thanks in advance!