There is no easy way to do this. Labels are set up as a table. If the
different label format you want to use has the same number of labels across,
you might be able to copy paste the text into the new format, but if there
is more than one page of labels, it is likely that the labels are the result
of a mail merge and will have section breaks between pages, meaning that
each page is a separate table. You could remove all the section breaks (Find
^ b and replace with nothing), then convert the resulting single table to
text, then back to a table with the right number of columns, then try to
paste into the label format, but this will probably give disappointing
results.
Your best bet is to create new labels with a mail merge. If the old labels
were merged and you can find the data source, you're in business. If not,
you can create a data source from the labels. There are various ways to do
this, but see
http://www.word.mvps.org/FAQs/MailMerge/ConvertAdrsToDatafile.htm for one
approach.