If you can afford it, a faster NAS would be more suitable for a 24 TB volume.
That said, there are two ways to do this:
Remove all the drives, insert the new ones. Rebuild the NAS, restore data from the backup
Incrementally expand one drive at a time.
The first way is much faster - requiring about 32 TB of disk I/O to create the array. The second method requires about 100 TB of disk I/O.
Either way, I recommend updating (or creating) your backup before doing this. The first method requires a backup. There is a lot of disk i/o with the second method, and your data will be lost if a disk fails during the expansion.
The incremental process is to hot-swap one drive at a time (NAS running). Wait for each resync to complete before you hot-swap the next.
If the volume doesn't expand after the second drive resyncs, then reboot the NAS - that should trigger it. Wait for the expansion to finish before moving on to drive 3.
Each expansion will take more time than the previous one (all sectors on all the disks need to be read or written during each expansion). The last one will take about twice as long as the first one. The NAS performance will be much slower than usual while the expansion is going on.
