Why did the delegates want to write a new plan instead of making changes to the articles of confederation?

Some thought that revising it still wouldn't make the national government strong enough. They needed to draft a new plan that gave the national government more power yet kept it from getting too strong with checks and balances.  

Assemblies do not have written codes of laws. laws are matters for magistrates and courts, not assemblies. codes of law are things such as civil law, criminal law, and constitutional law. what assemblies have are procedures regarding debating, voting and other business they might carry out.  also note that rome had three assemblies: the assembly of the soldiers, the assembly of the tribes, and the plebeian councils.