Paul Corfield wrote on 01 September 2013 20:21:28 ...

[snip]
The Northern Line is a very complex operation and you need to get test
trains running through both junctions and all of the branches to make
sure you are giving the new system a robust "check". RATP did not
convert their Line 1 to ATO without line closures and late starts
*every* Sunday for months.

The recent operation on Line 1 in Paris has been to convert from
driver-controlled ATO to driverless automatic operation using new
rolling stock, after installing platform edge doors at all stations.
For some months, the new driverless trains were mixed in with normal ATO
trains. It's now all driverless.

The Paris Metro converted most of its lines to ATO between 1969 and
1979, retrofitting ATO to existing stock. I don't know what line
closures this entailed.
--
Richard J.
(to email me, swap 'uk' and 'yon' in address)