Keep values from source cell even if source cell is deleted

Hi everybody.

I´m working in a workbook to keep tracking of the changes in the clients demand dates.

I have a initial date (from) and end date (to) and i need to store this dates when they are deleted.
I was able to come around a way to store the values and if the source change, the cell keeps the original value and a cell next to it keeps the new date, and so on, up to 10 changes.
I also record the date and time the change was made
But if i delete the source, everything is lost.
I need to figure out a way to do keep the stored values even when i delete the source.

Example of my sheet:

B2: source start date (from)
C2: source end date (to)

hidden collumns:

D2: initial start date =IF($B2="","",IF(D2="",$B2,D2))
E2: initial end date =IF($C2="","",IF(E2="",$C2,E2))
F2: 1st input =IF(E2="","",IF(F2="",NOW(),F2))

Changes:
start date 1: =IF(D2="","",IF(D2=$B2,"",IF(G2="",$B2,G2)))
End date 1: =IF(E2="","",IF(E2=$C2,"",IF(H2="",$C2,H2)))
Change time 1: =IF(H2="","",IF(I2="",NOW(),I2))

and so on up to change 10

Does anyone have an idea?