The thing the document buries
The document creates exactly three license options (narrowband, 3/3, 5/5) but never states how many existing operators currently hold licenses in this band or how they transition to the new framework.
The last band to get flexible.
Spectrum reallocation from rigid single-use licensing to flexible-use rules has been the FCC's slow-motion project since at least the early 2000s, when the commission began unwinding similarly rigid rules in other bands. The 900 MHz band simply outlasted the reform wave longer than most.
The same rule, another band.
The FCC has been converting legacy narrowband allocations to flexible-use licensing across multiple bands over the past two decades; this completes the same logic applied earlier to bands like 700 MHz and AWS, where rigid rules were replaced with market-structured license options.