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.
Although the file version is changed to 5.5.0, the Web UI says it's V5.4.x and allows to upload any 5.4.x through...