This is more of an EasyEDA question then a KiCad question. KiCad can not save to older file format versions, and older KiCad versions can not read file formats from the future. As a result, you are stuck with whatever EasyEDA has as import functions.
In KiCad it is doable to reverse-engineer a PCB from a set of gerber files. You can get 80% done in a few minutes but the last 20% may need a few hours of cleanup, mostly because a lot of information in the gerber files is simply not present. I do not know if EasyEDA has an equivalent functionality.
In KiCad you can also reverse-engineer any schematic by importing it as a bitmap (for example .png or .jpg formats). This is mostly boring work, but data re-entry is still a lot quicker then designing a circuit. When designing, you have to read and compare datasheets, decide which parts to use, decide how to connect those parts, calculate values for passives, etc. Having the original as a background image makes this work a lit less boring and quicker, and it reduces the chance for errors. Background images can be loaded both in the Schematic and in the PCB editors. This method is far from ideal, but better then nothing. And again, I don't know whether EasyEDA has something similar.