The exchange is made only if both Serene Master and the target creature are on the battlefield when the ability resolves.

Any power-modifying effects, counters, Auras, or Equipment will apply to the creatures’ new powers. For example, say Serene Master is enchanted with Lightning Talons, which gives it +3/+0, and it blocks a 5/5 creature. After the exchange, Serene Master would be an 8/2 creature (its power became 5, which was then modified by Lightning Talons), and the other creature would be 3/5.

If Serene Master blocks multiple creatures, its ability will trigger only once, and it will exchange power with only the target creature. (Note that Serene Master can’t block multiple creatures normally.)

When Serene Master’s ability resolves, its power becomes equal to the former power of the target creature. At the same time, that creature’s power will become equal to Serene Master’s former power.