Due to historical reasons, have a long formated sequential (text) file of
data. Need to re-write one of the early header cards. Tried the open and
rewrite scheme below, but rest of file disappears after rewriting the
required line.
So, short of copying the rest of the file, how can one do this?

File needs to be sequential & formated:

The open is

open( iU, file = cFileName )
write(iU,'(a)') ' VERSION_1 '

write(iU,'(a)') ' some text'

write(iU,'(79a)') cTitle

write(iU,*) rReal, iDum1, iDum2, iDum3

write 1000's of lines of data.

Now, need to change line 4 and keep all data following line 4. The
following deletes all data after line 4.

rewind( iU )

do i = 1, 3



read(iU,*) rReal, iDum1, iDum2, iDum3


iDum3 = iNewValue ! change parameter value

write(iU,*) rReal, iDum1, iDum2, iDum3


So, how to change only one line of this data file without reading /
rewriting the entire file.