How to find latest child for a given parent in same table?

Question

More details:
Here I am giving an example -
Orig_ID     Chg_to_ID
A     B
B     C
C     D
D     Null
From source I use to get Orig_ID values, using Orig_ID value have to find latest Child(Chg_to_ID) to that at any level.
Latest child identify by Null value in the column "Chg_to_ID".
If I get B --> need to replace B with D. I may get any value A or B or C or D -- but at an end I have to replace those values with 'D', since it is latest for any of those parents.
I tried to use "Connect By Prior", but it is tooo slow ofcourse query not returned after 10min also. Plz advise.

Answer 1

First time every employee will be assingend an ID and may changed/merged to another ID basing on some other conditins. So, any time first assigned ID may changed to new ID and will be updated to another column "Chg_to_ID" as winner ID and at the same time one more records will be created with new ID as ID and Null in "Chg_to_ID".
In my first post, given an example, here pasting again:
Orig_ID     Chg_to_ID
A-------------->B
B-------------->C
C-------------->D
D-------------->Null
In detail: One employee first assigned ID as "A" after wards changed to "B". At that time data would be like below
Orig_ID     Chg_to_ID
A-------------->B
B-------------->Null
After that B change to C, now it looks like this:
Orig_ID     Chg_to_ID
A-------------->B
B-------------->C
C-------------> Null
For each change one record will be inserted.
My requirement is -- Orig_ID may get A or B or C, but I need to replace those with "C", since it is latest child. For latest child "Chg_to_ID" is NULL.
Plz let me know for any clarifications.