Most fields give us “cleared for takeoff, switch to departure” at the hold short. The “no switches below 2500’” remark is probably in response to the SFA (single frequency approach”); however, I think that’s only while on an instrument approach, as I’ve definitely gotten the switch while on the visual arrival. In IMC, I’ve had “contact tower” inside the FAF and “contact tower on the rollout”, so it’s definitely not standardized that I can tell (maybe on a military airfield only?).
Usually internal comms on ATC freq are usually reserved for if an aircraft is single-comm, meaning one radio (of the two onboard) on one of the aircraft is inop. The caveat to that is if you get a freq change from ATC, often times the flight lead will add “flight GO” into his response to ATC to avoid having to say it on his aux radio (tactical freq that his flight using for internal comms). You’ll also often hear the flight check into a freq too: “Doodad 11” “Doodad 12” “Dept Doodad checking in flight level umpty-fratz” to make sure the wingman made the switch to the proper freq .
As far as if it’s easier to change freqs on military aircraft radios than it used to be, it really depends on the aircraft. Some aircraft require you to change the freqs on the radios themselves (toggles switches under each digit), in the AV-8, you can change the freq using the keypad (“upfront control”) on the HUD.
