If you're not planning to do this via MySQL. Then I'd probably recommend the following add-ons to assist you in the process:
Andrew Weaver's AJW Export This will allow you to export your
items from the channels you want to remove as .csv or .xml to
re-import back into the system.
Andrew Weaver's DataGrab This will allow you to map the data
back into the system.
Alternatives for exporting would be Eric Lambs, Export It. And an Alternative for importing would be Solspace's Importer.
I frequently just do MySQL queries, but when the jobs quick or needs to be repeated regularly, I admit Andrew and his company BrandNewBox have been a godsend. It's like we share some data cleaning and migration synergy.
Don't be afraid to add a spreadsheet in there to check that all the data came out properly formatted and ready to import back into the system. But overall, Andrew's stuff is easy to set-up, easy to use.