People who already hold HBD can sell it immediately and initiate a conversion to get it back (with an expected profit) in three days.

That's an interesting point, but I admit I'm a bit concerned that a lot of that HBD may be held by "uninformed users" that aren't aware of this possibility. Part of that is just that I have absolutely no idea who is really holding most HBD at any given time, since it typically seems to be mostly held on exchanges. And I have the vague feeling that most of these holders don't really track what's happening on Hive.

I guess I'm still in favor of setting the new conversion time lower than the reverse conversion time, and seeing how that goes. Maybe 24 hours would be a good starting point. I also need to doublecheck that adding the delay won't complicate the implementation of the new conversion too much (my guess is that it won't, but best to be sure).

EDIT: as a side note, BitShares used a 24 hour delay for it's conversion time, and it never lead to any problems. Although it's worth noting that it also had a cap on how much could be converted in one day (not that I want to implement that, just trying to mention all pertinent details).