I am no expert, but I thought they could bid to be a plumber no matter what, as long as there are still under 60 S/Os that are junior to them.
If you are referring to new hired assigned an FDA, no. In order to downbid they would need to wait 24 months AND there would have to be a bid with primary or secondary vacancies that they could bid and hold in the 727 SO seat. If the Company was to hire guys into the S/O seat prior to the FDA guys being eligible to bid, they would be essentially locked into the FDA until the next bid.
Any new hire to an FDA should plan on 4-5 years(minimum) in my opinion to get any seat back in the US.
They will more likely be able to get back in the right seat of the 72 sooner than the back seat, unless age 60 changes and the retro language survives.