Let's get the whole picture:
before the move:
P announces A'/L1 to <the world>
B announces A/L2 to P (where L2 > L1)
C announces B/24 to B
There is no reason for P to announce anything more specific than A'/L1
to the rest of the world unless B is multihomed. If there is no A'/L1
aggregate then never mind.
after the move (if B is single homed to P):
P announces A'/L1 to <the world>
B announces A/L2 to P (where L2 > L1, L2 < 24)
C announces B/24 to B
There is no need for P to announce anything more specific than A'/L1
if B is not multihomed. If A'/L1 doesn't exist, there is still no
reason to announce anything more specific than A/L2.
if B is multihomed:
P announces A'/L1 to <the world>
B announces A/L2 to P (where L2 > L1, L2 < 24)
B announces A/L2 to <other provider O>
P announces A/L2 to <the world>
O announces A/L2 to <the world>
C announces B/24 to B
P announces B/24 to <the world>
The last part is unfortunate.
Curtis