Spot on, the upgrades involve massive infrastructure works at the technical sites/headends. Changes from the legacy UBR10K and BSR64K to the newer Arris E6000 and Cisco CBR8 are part of those works. Utilisation can be a huge problem as well so even a technical upgrade to a new CMTS might not allow 300Mb launch in an area as further node segmentation could be required. Perversely, 300Mb can be offer in areas that haven't had an upgrade if there is suitable capacity and low utilisation, these speeds can be offered on legacy CMTS areas.
Its all very complicated