You probably can! At least if your FCs are at least F4/F7 - F3 series microcontrollers are no longer supported. But the way Betaflight 'upgrades' work you'll need to reconfigure the FC after the upgrade. And using the CLI dump made with an older firmware version isn't supported - some options go away or get renamed, PID tuning options in particular should not be blindly copied between versions.

O3 is the best quality digital video system (made by DJI). The air unit (camera+digital vtx) is $230, and the goggles are $500. It can also record up to 4K at 120 FPS (in-goggle max quality is 1080p at 100FPS). Analog is definitely way cheaper (at least the vtx side), but I think most people who could afford digital have switched over the last couple of years.

DarwinFPV BabyApe is $80 and flies better than anything you’ve built 4 years ago due to improvements in Betaflight. Sure, if you want O3 you need to pay more.