I just found a way how to downgrade 5.5.0 to 5.4.x (it worked on DS-2CD2142FWD-I at least).
In short, use hikpack to change 5.4.x's version to 5.5.0 and then upload it via tftp.
Just wanted to thank you and let anyone else know this procedure also worked for 3 of my DS-2CD2142FWD-IWS Cameras that I had upgraded to 5.5.0. I used hikvision-tftpd/hikvision_tftpd.py at master · scottlamb/hikvision-tftpd · GitHub for the actual restore from OS X. The only think was that I had to cut power to them once more after downgrade and then they came back with the default 192.168.1.64 address.
	
). These instructions are not really detailed because I'm not sure if that's an universal way or I'm just lucky and that works just for my camera.