Yep, if that happens you are left with two choices. One you mention of with digging in water, and I still would leave it separate till done.
The second, which really isn't likely doable, is to pump it out of the new dig into the old pond. Your pump will have to exceed the flow thru rate. Means LOTS of pumping and you can never give it a moments rest till its all dug.... digging like mad. Means big equipment, and possible multiple guys working, to get it done fast.
