Imagine that this situation happened when 99% of the coins were mined and suddenly 10% of coins were going to be given to miners, giving them 10x as much as before this decision.
Doesn't make much sense to me.
Edit: The change that does the least change in incentives is to announce that there will be a window of spendability that reaches X years back at all times, where X > the oldest coin. Then no coin is invalidated now, but a rule that keep uncertainty bounded has been introduced.
This is wishful thinking. If you really want to solve this problem, require that anyone who wants posting privileges post a bond, in Bitcoins. Some nominal fee like $25 worth of Bitcoins. If they break the forum rules, like posting something that could have been found via search, they forfeit the bond. Problem solved.