There is no 'flashing' to metro.... whoever you talked to is stupid.
You get the SPC (also called MSL), open up EPST by dialing ##778, enter the SPC, then go in and change your MSID, HDR, PPP, S.IP, MIN1, MIN2, MCC, MNC, NMSI, IMSI... and I think that's it. Most of those are just your new phone number, or
phonenumber@somethingmetropcs.com.
It's actually really easy (at least I think, I've never done it).
But yeah, whoever was trying to flash this like an LG phone is a tool.