progression of the expander 1 through the casing 32 after the casing 32 has been expanded the expander 1 is removed from the casing and the retrieval tool 46 is lowered on running string 48 through the casing 32 upon arrival of the retrieval tool 46 at the sleeve 36 lowering is slowly continued until the retrieval tool latches to the sleeve 36 by virtue of latching of the spring loaded pins 50 into the openings 52 of the sleeve 36 the retrieval tool 46 is then pulled upwardly on running string 48 as shown in fig7 b the sleeve 36 is thereby radially compressed as it moves upwardly into the remainder casing portion 41 compression of the sleeve 36 does not require a high compression force since such compression is accomplished by closing of the slots 50 of the sleeve 36 furthermore the tendency of the sleeve to spring back elastically and the pulling force exerted to the sleeve by the retrieval tool enable easy removal of the sleeve 36 from the casing 32 the sleeve 36 is finally removed from the casing 32 at the upper end thereof in this manner it is achieved that the lower portion of the casing 32 is expanded to a larger diameter than the remainder of the casing so that a subsequent casing not shown can be installed and expanded below the casing 32 whereby an upper end portion of the subsequent casing extends into the bell portion 38.