The problems a
1) with large rhododendrons it is very difficult to move without greatly
disturbing the root system.
2) summer is a time when the plant needs much water.
3) it is difficult for a damaged root system to secure enough water for
the plant.

Steve's cautions are to be heeded, but I think it also depends on where you
live and where you will relocate the plant.

Up here in the Pacific Northwest (a rather Americo-centric term, as for those
of us in Canada, it is really our Pacific Southwest), we can transplant right
into the summer as long as we don't stick the poor plant in the full sun, where
as Steve points out, you may either give it too little water, in which case it
will die, or too much water, in which case it will get root rot - and then die.

It is easier to get a balance when the plant has some shade than if it were
trying to make do in full sun.

The sort of rootball it has also makes a difference. A pot bound plant will
need the outside roots teased out or it can just sit in a tight ball for years
before dying.

One guy I bought large plants from when he moved had an interesting method of
planting. He laid down plastic sheet to keep down weeds, and then placed the
rhodos on top in a few inches of bark mulch. They thrived, and ended up with
flat root 'balls' often the size of bath mats, and could be moved so easily
that they probably didn't even know they'd been transplanted.

Just don't make the mistake that many people do and plant the poor things too
deeply. Often almost placing them on the surface and mounding bark mulch around
them is adequate, though most people dig a shallow planting hole. Digging deep
can result in lack of drainage and another dead plant.