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They haven't changed engines. Lumberyard is just a fork of CryEngine 3.8

They had to adapt the changes they had made to CryEngine to Lumberyard which were different versions of CryEngine if I recall correctly.

CIG had stopped implementing new CryEngine changes at 3.7. So they just had to include the changes from 3.7 to 3.8(where Lumberyard forked).

They said it took 3 engineers one weekend of work, using a diffchecker.