Renaming of labels, as far as I know can't be done on the N900. Label names are not saved in vcard .vcf files and therefore can't be transfered to another device, this could be why it's not commonly done.

Save a ladel modified contact vcard from your symbian device and you will see no ladel name, the vcard format just doesn't support it. Annoying I know, you'll have to save each contact multiple times if the three options aren't enough or try to learn the different numbers.

What is more annoying, there is a vcard standard, but phone manufactures all treat it differently with the fields they will support.