Unless they are using some temporary dam a fair length of the canal will have to be drained to do the work. Their planned schedule is during the Stockport Primary Schools Easter holiday, but it shouldn't interfere with the Locks Festival, which is in July.
On a separate note the CRT has closed off (in theory, if not entirely in practice) the Macclesfield Canal towpath as it enters Marple, so it is quite hard to walk any of Marple's canals at present.