Hi Sirius - it is actually possible to have the surface update in response to editing of the curves.

To do that you have to keep the original curves (don't delete or move them), and then select the surface that was generated by the Construct > Planar command, and enable history updates on it. You do that by using the Edit > History command, and click the "Enable update" button that appears in the command options area.

Once updates are enabled the planar surface will get recalculated when the original input curves are edited.

But really for a planar surface it's also pretty easy to just delete it and recreate it - the history update mechanism tends to be more useful with things like a revolve or a loft where you kind of want to see how the surface shape changes in response to your curve edits. In the case of a planar object it's much easier to understand how the final surface will look since it's all flat, so there isn't really as much benefit to doing the history thing with it.

Some commands have history updating turned on for their outputs automatically, like loft, revolve, and mirror, but other ones have it turned off by default since it can sometimes be a bit surprising in some cases to have the updates applied automatically - for those cases you can turn it on by that Edit > History command. You run that command on the output object (not on the input objects).

History is limited in other ways though as well - you need to have all the original inputs that went into the command still present in the current model so you can't do things like delete the original input objects. Also some commands are the equivalent of deleting an object and replacing it with a different one such as booleans, so doing booleans on objects will break history the same as deleting the original input objects will.

In the future I want to make an extended mechanism for preserving history through more operations - that will probably involve keeping some kind of data store of previous versions of objects around, which may grow to be pretty large in size.

- Michael