If you have photoshop. you can simply just export and OW from Overworld Editor. Open the exported OW in photoshop. go to Image - > Mode - > Color Table and edit pallets there. save the ow with new paletts and import it back to game in OWeditor and don't forget to import paletts.
Other way is with APE. You'll have to find the offset of the OW you want to change. and change the pletts with APE.