It isn't really a cost/problem to "ask the whole population to replace their coins". Coins are constantly being circulated, with worn ones replaced by new ones

It isn't really a cost/problem to "ask the whole population to replace their coins". Coins are constantly being circulated, with worn ones replaced by new ones anyway. The cost of changing comes primarily from businesses having to update vending machines, coin counting machines, etc (as well as increased production costs due to the new security features.) But this switch has been planned for many years, so businesses have had a long time to prepare.