Unless the volume is at the dedupe limit you can just go ahead and resize the volume. There is no need to undo the existing dedupe data unless it's at the dedupe limit or you think your read performance for that volume has been degraded by deduplication. And yes, sis undo will clear the asis data but have a look for any open BURTs against the release of ONTAP you're running - there have been some buggy versions. Oh, and make sure there is sufficient room in the aggregate before running sis undo for that volume. You'll have to look at the documentation for you specific version of ONTAP as many restrictions are removed and limits increased in newer versions.
Another option you may consider is rerun ASIS on the volume with the -s switch. This will start dedupe fresh and rebuild the database. If the database is large it may contain a large number of stale fingerprints and rerunning dedupe from scratch will free up a bunch of this space. Just be sure you have the free space and the cpu cycles to have dedupe run for a while as this will be a complete scan and may take a while.