Here's some what-if's to enter into your proposed method...
- The existing transformer is for lighting and convenience power. Now you gotta bring provide or rig some other means for light to work by.
- The transformer is set on a housekeeping pad. The duct lift's base support struts do not fit around the pad.
charlietuna's estimated time is three hours with a schedule shutdown after business hours... properly prepared. I can say I have set a transformer (wall-mounted above switchgear) with chainfalls, during business hours without interupting power to anything. This includes setting the chainfalls, rigging the transformer, hoisting it, anchoring the thru-wall bracket, lowering the transformer and bolting it to the wall bracket. And this is solo... no one helping me. Two man-hours max with everything needed on site.
Granted the slowest part is the hoisting with chainfalls. What's ten minutes? Who's trying to reinvent the wheel?