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.