Hi john66,
I switched your post to a new topic of its own so your questions and any replies wouldn’t get mixed in with another members questions.
There are several potential issues with replacing a waterbed with a regular mattress as a waterbed “insert”
First of all I would make sure that your waterbed frame is a suitable size for the mattress you are purchasing and that the mattress doesn’t fit too tightly inside the waterbed frame so that there is some space around the mattress when it is inside the frame and there is enough room around the mattress to put your hands in between the mattress and the bedframe to add sheets and bedding and to make your bed. In many cases this may require a custom size that is a little smaller than your waterbed frame.
Second is that if the mattress fits tightly inside the bedframe there there would be a greater risk of moisture and humidity retention with a foam mattress inside a waterbed frame that doesn’t allow any airflow around and underneath the mattress so it could add to the risk of developing mold, mildew, or dust mites (see post #10 here). This could be more important if there are also other risk factors involved in your sleeping environment.
I would also make sure that your mattress is thick enough so that it’s higher than the waterbed side rails so that it’s easier to get in and out of bed or sleep on the edge of the mattress without feeling the hard sides of the bedframe.
Phoenix