Change the traction package on a unit, and put it back in service.
Don't see why that would require closing the line. It might not
be possible to couple a DC unit and an AC unit in a single train,
but they can certainly share the same track and power rails.

If they do it lets hope they don't reduce the performance of the
central line trains to the slug like performance of the northern line
ones. The central trains are the only ones on the tube that have
anything approaching the decent acceleration that you'd expect on a
modern metro.

B2003