Hey man, I am using LG UI 3.0 theme with 6.39(O) rom. Will anything lost after flashing 6.4?
You will loose everything related to the new theme... v6.4 is a compete ROM.
But you probably can flash the theme again over 6.4... ask your theme developer. Changes between 6.39 and 6.4 are kernel, ramdisk, zeustweaker, xperia and init.d related... there's no theme modifications.