Looking at about a day and 2-3 head to mark and unwire and pull back everything, hopefully a day to disassemble and remove. I’d say one day for all of this if not for the number of sections. Usually we knock this all out on the first day unless there are complications like difficult access.
At this point you want to dowel in rebar, form, and pour any base modifications with high early strength concrete so that it has overnight to set.
Then a day to put the new stuff in, again 2-3 head. After this looking at about 4 hours per section to land wiring and terminate for say 4/0 or smaller and it gets up to about a day for multiple 500s with 1-2 per section. Then figure in your labor with multiple sections. If it’s just landing wires sure you can do it in a couple hours but if you have to splice, cut, and/or rework terminations you can quickly turn this into an hour per cable, assuming you have lugs and cold shrink or tape termination kits on hand. Trust me, you ALWAYS have to at a minimum redo a few terminations where the silicone cracks and starts peeling back.
One man, one day to set up relays. Mostly the two mains, the tie (if it’s separate), and the feeders are your three sets of settings. Everything else is copy/paste and change things like the tap for long term tripping.
Two men one day if you are doing extensive NETA style acceptance testing. If you are just going to Megger then don’t count this at all.
Two men one day for commissioning/startup, assuming the plant doesn’t slow you down. When they take an hour or two per breaker test it can get obnoxiously long.
Two men half a day for cleanup.
This is assuming experienced labor. In all honesty the number of sections doesn’t matter that much. It’s all in the issues you run into and if you can flex up the labor on wiring then downgrade to normal at the beginning and end. Most of the time these things take about a week, maybe two with a truly big system.
Keep in mind a lot depends on the condition of the original equipment too. If it’s structurally in rough shape just replace. But if breakers or equipment are available, refurbishment or retrofit/retrofill is a lot more economical. It could be a simple matter of swapping out say old with new or making doors with new relays and simply swapping out all the old induction disc units with microprocessor relays. There are some breaker designs that are inherently awful and others where it’s just lack of maintenance and old electronics that are the limiting factors.