Rather than appending a rule to a chain, you want to insert or replace one elsewhere in the chain.
Instead of the -A option, use -I to insert or -R to replace. You'll need to know the numeric position, within the existing rules, of the new rule. For instance, to insert a new rule in the fourth position in the chain:
# iptables -I chain 4 ...specification... # ipchains -I chain 4 ...specification...
To replace the second rule in a chain:
# iptables -R chain 2 ...specification... # ipchains -R chain 2 ...specification...
When you insert a rule at position N in a chain, the old rule N becomes rule N+1, rule N+1 becomes rule N+2, and so on. To see the rules in a chain in order, so you can determine the right numeric offset, list the chain with -L. [Recipe 2.16]