I think I have to disagree on the adjective 'minor' in the above. I don't dispute the several advantages noted, but it's not a minor change due to the unmentioned disadvantage(s).
The primary disadvantage depends on how you use the forum. I look at 'New posts'. I never look at anything except 'New posts'. I suspect a fair few people do this. Is it the case that an edited post will appear in that list? I think not but happy to be corrected. Assuming that edited posts don't qualify as 'new' for this purpose, I'm never going to see participants' updates unless I go and find the thread and scroll up and down it to see whether people have added a ride for the month. That's a great deal more 'effort', and certainly more time, than simply noticing the thread reappear and clicking on it, which takes me straight to the latest addition. So, actually, having thought about it, I disagree with the 'saves us all an awful lot of scrolling..' thing too.
To me this will make the challenge less interactive as I simply won't see things much and won't have much sense of things happening. Yes, clearly I can go and scroll up and down twenty or so posts each month and see who has completed, but I suspect I'll do so with considerably less alacrity than the current situation, where I tend to see new rides immediately. The same applies to multiple rides being recorded in fact: it's more dynamic with people doing more. Entertaining to see how many 100s
@GuyBoden has done in the last week, for example ;-)