is it possible to make certain cells required based on criteria

[Mim]

I am trying to lock and unlock certain cells based on data input into a cell.
Is this possible?
Example: If user types "New" into Cell A1, then lock Cells A2 and A3, and
put cursor into Cell A2 ready for next input.
If user types "Existing" into Cell A1, then unlock Cells A2
and A3, but lock Cells A4 and A5. Then put cursor into Cell A4 ready for next
input.

Thanks