I did what you are thinking and it doesn't always work out as you think when growing zoas on the sandbed.
Once the zoas migrate from the plug to the sandbed, they will continue to grow and do so. This is especially true for fast growing zoas.
However, before the zoas are actually growing on the sand (feet attached to the sand), the zoas seem to be very hesitant to spreading on the sand. They will grow on and around the plug for as long as possible, to the point of having their feet UNDER the plug. I have fairly large colonies of over 50 heads and assume that some are one the sand, but all are actually still attached to the plug.
If you are interested in easily fragging, I suggest cutting thin wafers from dry rock, attaching the zoas on to that substrate and then, putting that on the sand bed if you prefer to have them there.
On a side note, my zoas seems to like growing off from the plug and attaching themselves onto the egg crates in my frag tank. This is opposite of what I want. They seem to love egg crates.