At Coxwell or something like that, it's not pretty but it's certainly feasible to close most of the entrances/exits, and funnel people through a little temporary detour entrance. At most of the downtown U stations (especially Union, King, Dundas, and Bloor/Yonge) that is simply impossible due to the passenger volumes at the farelines. The only feasible strategy given the passenger volumes in play is to close down only portions of the turnstiles while leaving others open, and complete the installations piecemeal, assigning small teams to a large number of stations over a longer period of time, instead of blitzing the installation at one station at a time and closing most of it.
All of the downtown U stations, if I recall correctly, either have two opposing lines of turnstiles or in a line of turnstiles they have a collector's booth in the middle, so in any of those stations they can just close one set and leave the other open, and that's in one given entrance, no need (generally) for closing one entrance and rerouting people to the other.
Union, while extremely busy at the farelines due to the PATH and GO, is largely easier since it has the two large farelines, and the south one is even large enough to do in halves.
Of course, this all depends on the TTC putting enough thought into this...