I would use a trash pump. It will be a pain to keep setting it up, but I think that would be best. However, I would consult the guy who going to do the work. He may plan on cutting it open anyway ( which I would question the guy's pond ability at that point). 1/2 pond would drain quickly with a 3 to 4" trash pump. You may want to dig a pit for the suction hose to be below the mucky level. That will be the only way the muck will begin to dry out, for the water water to flow into this pit out of the muck.
I am a little concerned for you about the water supply for a bigger pond. You say that there is no real overflow on the current pond. This tells me that rains rarely fill it up enough to need a regular overflow system. How will you get more water to fill a bigger pond?
