Upgrading existing Firefox Portable stable version (release update channel) installation to Firefox Portable Beta !

but probably kind of 'by hand'

The beta versions install itself into separate folder, just so that hey do not interfere with the actual release version.

So you could make an install of the beta version, this will land in a special folder with something like 'test' in its name.

You could then either copy your profile to this new folder , it is in the data folder and goes into the data folder.
This will bring all personal variables with it, all extensions and bookmarks etc.
Note that number of extensions will need to be 'updated' after such move, but they might all work after that.

The other version would be to copy contents of the other folders (except profile) from the test folder to the release folder of your firefox.

I would prefer the first version to test if all works before version 2.