Existing LPARs with dedicated memory can easily be converted to shared memory. To do this, however, an LPAR must be shut down. The change must be performed in the profile of the LPAR. Only the attribute mem_mode has to be changed from ded to shared. The LPAR can then be reactivated with the changed profile and then immediately uses shared memory.
The procedure is shown briefly using the LPAR aix22 (located on the managed system ms03):

1. Shut down the LPAR:

aix22 # shutdown

SHUTDOWN PROGRAM
Mon May  3 11:57:07 CEST 2021
Broadcast message from root@aix22 (tty) at 11:57:07 ... 
PLEASE LOG OFF NOW ! ! !
All processes will be killed in 1 minute.
Broadcast message from root@aix22 (vty0) at 11:58:07 ... 
! ! ! SYSTEM BEING BROUGHT DOWN NOW ! ! !
Stopping IBM.ConfigRM...
0513-044 The IBM.ConfigRM Subsystem was requested to stop.
…
Unmounting the file systems...
Unmounting the file systems...
Bringing down network interfaces: en0 lo0
open: No such file or directory
May  3 11:58:35 portmap: terminating on signal.
....Halt completed….

Alternatively, the system can also be shut down using the command “lpar osshutdown“:

$ lpar osshutdown aix22
$

2. Changing the memory mode in the profile of the LPAR (here standard):

$ lpar -p standard chmem aix22 mem_mode=shared
$

3. Activating the LPAR with the changed profile:

$ lpar -p standard activate aix22
$

4. Verification:

$ lpar lsmem aix22
             MEMORY          MEMORY         HUGE_PAGES    IO_ENTITLED_MEM      PAGING_VIOS
LPAR_NAME  MODE    AME  MIN   CURR  MAX   MIN  CURR  MAX  AUTO     CURR    PRIMARY    SECONDARY
aix22      shared  0.0  1024  2048  8192  0    0     0    1        351     ms03-vio1  -
$

$ ms lspgdev ms03
                                                                  REDUNDANT
MS_NAME  DEVICE_NAME  PAGING_VIOS_NAME  STATE   DEVICE_NAME  PAGING_VIOS_NAME  STATE  SIZE   TYPE  LPAR_ID
ms03     hdisk4       ms03-vio1         Active  -            -                 -      51200  phys  5
ms03     hdisk3       ms03-vio1         Active  -            -                 -      51200  phys  6
$