As someone in FSS who tried and then gave up on ever getting back to ATS (I transferred voluntarily to FSS), you are ineligible to apply to bids UNLESS you are out of the agency for more than a year. This is per top ATO management to my senator when I sent my complaint about lack of progress after 3 years is fighting with the FAA and NATCA. The RVP of the Alaska Region was informed by a facrep that because of the union's lack of progress with my greviance on the NCEPT excluding FSS dated April of 2018, there are people looking at leaving the agency. What was the RVP's response? "I have sympathy, but no empathy. If they want to quit, let them quit" per multiple people on the TELCON. The union has zero care about FSS and the FAA has less than that. If controlling is your end goal, FSS will only ruin the chances... even if the best case scenario happens, which per the National FSS Rep in NATCA is a minimum 3-5 more years for NCEPT to implement FSS.
Good luck, but if you want back, you'll need to leave the agency first. 🙁