Make sure the parent directory is your current directory and you can use this command:
exiftool "-directory<EXIF:DateTimeOriginal" -d "%Y%m%d/%%d" -r .

Example, given this tree \f for the current directory, then running the above command, gives the end result:

X:\!temp\test>tree /f
Folder PATH listing for volume Data
Volume serial number is 00000200 1CFF:E96C
X:.
│   20140404_040404.jpg
│   20150303_030303.jpg
│   
└───2010
    └───12
        └───25
                Test.jpg


X:\!temp\test>exiftool  -d "%Y%m%d/%%d" "-directory<DateTimeOriginal"  -r .
    4 directories scanned
    3 directories created
    3 image files updated

X:\!temp\test>tree /f
Folder PATH listing for volume Data
Volume serial number is 00000200 1CFF:E96C
X:.
│   
├───2010
│   └───12
│       └───25
├───20101225
│   └───2010
│       └───12
│           └───25
│                   Test.jpg
│                   
├───20140404
│       20140404_040404.jpg
│       
└───20150303
        20150303_030303.jpg

Test.jpg still retains the 2010/12/25 path under the new directory. Also note that the original directory is not removed.

The addition of %%d escapes the last %d so it is interpreted as the directory from the -w option instead of the date from the -d option.

This command will result in different results if you aren't using the current directory. If the current directory is on a different drive than the target directory, then you will most likely get an error as it tries to append the target directory name onto the current directory, something like c:\CurrentDir\D:\TargetDir. If this needs to be done, then look into the %d options under the -w option.