Howto do a hard upgrade from postgis-1.5 to postgis-2.0
If you can't follow the correct procedures suggested by Mapperz and Andre Joost, I'd advise you to do an extraction through tools such as  or "Qgis" (ogr) in shp format, and then perform a batch import in a fresh postgis 2.0 installation with shp2pgsql-gui.

You just have to make sure that during the export value of the attributes (columns) of the various spatial layers may be slightly different because of shape format specification.

The length of the porting procedure depends strictly on the amount of data.