I remember that question and I remember this answer being given to me... that at some point during the "anointed" persons life if they became unfaithful, they could be removed from that 144,000 number and when that happens a SLOT OPENS UP!!!
If they are DFed they are no longer part of the 144,000 and a space opens up. However if they repent and are reinstated they are part of the annointed again. Then the person who filled the slot has to step down and go on a standby list for the next opening.
I don't think I am making this up .....honest.