Freighter conversions are (generally) not driven by the engineering of the conversion. 90% or more of the engineering has been accomplished for a passenger to freighter conversion. The main driver historically has been the residual value of the donor passenger aircraft. Historically, residual value drops sufficiently after 20 years to make passenger to freighter conversions cost effective. But used 777s are holding their value longer than usual, so there aren't enough used 777s available to make a conversion program cost effective.
As for replacing the floor beams, that's relatively straight forward. All the floor boards would need to be removed anyway and once they're out, access to the floor beams is not that difficult. Much more difficult, complex, and invasive is adding an upper deck freighter door.