I’m not certain, but on a commercial power plant they remove essentially the whole top of the reactor to pull and replace the rods. Not a trivial task even with full access, I can’t imagine how much of a pain in the neck it would be to feed the rods through a small hatch using remote TV and waldoes.
My understanding is that on commercial plants, they don’t remove the “whole top” rather that there is a mechanized process that removes the old fuel and then inserts new ones, usually from the top down, I think.