Which ROM?

I just bought an AT&T N6 from Swappa. I plan to use it on Fi when my invite comes, for now I will transfer my Straight Talk service to it. It came with LMY47Z on it, then downloaded an update and ended up on LMY48I. Now, LVY48E says it is "For Project Fi ONLY", which leaves me to ask, will I have to flash LVY48E when I switch to Fi? If I flash LVY48E now will it work on ST? Any helpful insights would be appreciated.