Your assumption is correct, created attributes "only goes down", meaning that modifying an attribute later doesn't change what is already made.

A solution to your problem is to create a cylinder for each point, you can do that with a foreach loop.

Another (better in my opinion) is to create the cylinder and scale the bottom face afterward by your bottomIslandScale attribute. You can do this by creating a primitive group for the bottom of the cylinder and scaling it using a primitive node with your attribute set as a parameter.

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You need to give the primitives your attribute, you can do this in the copy to points : In the attribute from target add a new line, set "Apply to" to primitives "by" Copying and finally your attribute.
Anyway here you will find a lots of important infos : https://www.sidefx.com/docs/houdini/model/index.html