It takes around four hours and is only done in Frankfurt.
They take out two rows of C class and install three rows of Y class. The seats are mounted on rails that run the length of the cabin, so this isn't complicated at all. There are enough O2 masks and reading lights installed underneath the bins for the Y configuration and the excess is covered up in C configuration.
While it is preferable to send out a C configured aircraft on a route that requires many C seats (and vice versa), occasionally demand for more Y seats on more routes requires a change. Thus a truck that looks like a gigantic catering car will appear next the plane and lift itself up to reach the second door from the front (approx. where the change of seats takes place).
LH had announced the configuration change, but never converted all aircraft. They just increased the number of routes where they planned to use F16/C66/Y270. When more C bookings come in, they will revert to F16/C80/Y234 before too many Y seats are sold, as they will change to the opposite if not enough demand for C materializes, so they can sell more Y seats.
Meanwhile they have permanently changed the first class layout, so the configs that can occur nowadays are F8/C80/Y234 and F8/C66/Y270. Next, they will permanently change the C class to fully flat (not angeled) and probably will again design the cabin such that two (or more) configurations fit.
The Airbusses have a similar arrangement, it is mostly used on the A340-300. F8/C48/Y165 or F8/C36/Y197. The ones without first class remain without first class permanently (C44Y222), occasionally an aircraft with first class is filling in but the hardware remains with the F seats, however for check in purposes the F seats count as C class (for the most premium passengers) and the config C54/Y165 or C44/Y233 will show up in the computers.