I've not seen / found a solution to this. It feels like a big hole in the commcare system, to be honest. You could just add a column to the xlsform call "case_variable" or something, which automatically assigns the case variable upon form creation.
Not having a fluid xlsform-type interface for building & editing forms is another huge gap that imo Commcare should invest in to bring it in line with it's competitors. The current xls form compatibility is janky.
sounds like something that only a developer would say
are you trying to ADD new case properties with an excel upload? or are you trying to edit your form(s) through a bulk upload?
remember that commcare uses xpath and xml.
there are a few things you can do if you really really want to use a text-based editor or something. such as editing the form's source XML.
you could also download an XML version of your form, edit that, and upload that again. but the thing with using this approach is that you use some of the quality of life functions introduced through the app editor if you don't use CommCare's conventions.
I have not personally yet found a reason to bulk edit forms...yet. I would love to learn about the requirement that made you think "it sure would be nice to be able to update forms with an excel file". Could you please share that?