Think of it as a project to be completed instead of one or more scripts to run. This is especially true if the system is doing some kind of production work.
Survey the netbsd system cataloging all the applications, configuration and data that the server needs to perform its designated tasks
Verify that freebsd can meet all of those needs.
Stand up two fresh vms with freebsd and start installing the need packages. Use one for testing, use the other for production.
Migrate the needed configurations/data from the old system to the test system.
Test. Test. Test.
Make sure you can integrate the server into your backup solutions or alter those as needed. It will also need to be added to whatever other support services it uses (monitoring, logging, etc).
Do it all over again for the production system when you've worked out the kinks in the test system.
Create a cut-over plan. Hopefully you can stand some down time, if not you will have to find a way to quiesce the current server so that it remains up but doesn't change while you are doing the cut-over.
Test the new server.
Migrate the needed configurations/data from the old system to the new system.
Monitor carefully.
