You can just add more songs to the original list in Strum Machine as usual. Anyone who has added the list to their account will have it updated, since they’re actually viewing your list rather than a copy.
That said, you bring up a good point: the list preview on the forum isn’t updated. That is, I can see that the list you posted still says “40 songs” but you’ve added 7 more to it. This is because the forum caches the list details, and doesn’t have any way of knowing that they’ve been updated.
This is a tricky one since the Strum Machine and this forum (powered by software called Discourse) are totally separate. I’ve written some code to integrate them – shared logins, previews when posting songs/lists – but there are limits to what I can do.
It does look like there’s a way to manually update the listing on the forum: you edit it (with the pencil icon) and then save it. This re-fetches the list from Strum Machine. False alarm – this doesn’t work. There is a way for me to tell the server to re-fetch the data though. In the future I may be able to run an automatic process to do this, say, weekly. I’ll look into it after the holidays.