According to the release notes for Data ONTAP 8.2.1 RC1 (7-mode), you can convert an aggregate to the 64-bit format without adding storage. Below is the excerpt from the release notes that answers your question:
"Aggregates can be expanded to 64-bit format without adding storage
Starting with Data ONTAP 8.2.1, you can expand a 32-bit aggregate to the 64-bit format without adding storage to the aggregate. Converting to the 64-bit format enables you to use Data ONTAP features that require 64-bit aggregates, such as compression and Flash Pool aggregates."
Re: upgrade 32->64 bit aggr/vols non-disruptively, without adding disk