That would depend on how they are removed.
If they give you 25 new dinar for that 25000 note, then it's a very big reduction.
If they give you one thousand new 25 dinar notes for that 25000 note, then that doesn't reduce currency in circulation by 1 single dinar.
Dow Jones Newswire: A current 25,000 Iraqi dinar banknote will become IQD25, and a dollar will equal only 1.17 dinars.