As someone who has transferred into DfTO, I can confirm:
You loose your First Group (or other owning group) passes and boxes and other benefits such as Friends and Family tickets, on the date of transfer, and get a strict letter saying that if you use your passes or boxes, even if dated to expire beyond the handover date, after the date of transfer then you could be subject to disciplinary action.
There is no DfTO reciprocal travel arrangements between other DfTO TOCS. So far, I am told, that all DfTO TOCS have agreed amongst themselves that they would like to have and are willing to have such a scheme, but the DfT itself has blocked it, allegedly saying it is too expensive.
All the above being said, in some cases where there were historical agreements between TOCS for certain benefits such as free residential travel on other TOCS services, these have remained even after transfer. And if the Friends and Family tickets are TOC specific (like Northern's were) then they will almost likely stay, rather then an owning group scheme (like the First Group ones) which will disappear when the owning group does.