Notepad++ is good for migrating and comparing configs using the compare plugin.
Sometimes it's necessary to migrate an old config from a C series to an E series. Depending on the models (where there is no overlap in firmware versions), it's helpful to have an intermediary D model to ensure you follow the recommended upgrade paths when migrating.
When changing the headers, ensure the values are the same or you will have odd problems when you try to login.
Map your interface names across. Some models have wan1, some have wan, some have ports labelled lan, internal or port.
Once you know the port mapping, find and replace all "wan1" with "wan" for example (including quotes) for each interface mapping.
Backup the config and reboot.
Check for any errors at each stage or end of the process with the command: diagnose debug config-error-log read
Use the Notepad++ compare plugin to see the differences between the original config and the new one.