Sometimes, if you have a large parcel shapefile, it can take too long to unbuild and rebuild and you may want to edit only a portion of the shapefile. If you have a large polygon theme and you have a specific area you want to add parcels to or change parcel boundaries in (like a new subdivision), you can unbuild just that area, change it, rebuild it, and merge it with the rest of the original shapefile. More detailed instructions are in the Help for the Merge Themes button on the Main Menu. It is particularly important, when doing this, to make sure you do not do any edits that could cause your new polygons to overlap with the original polygons. To prevent this, make sure that when you choose which polygons to unbuild, you select enough polygons so that you don't edit any of the shared boundaries between the polygons you unbuild to edit and the polygons you leave as polygons. It is better to err on the side of unbuilding too many polygons than to end up with overlaps. Also, when you rebuild and merge, make sure that in the 'original' theme you select only the polygons you didn't unbuild before you merge it with the newly edited and rebuilt shapefile (use the Swap selection function). All in all, you should unbuild and build the polygons as one big unit unless the shapefile is too big to build in a reasonable amount of time.
