As long as you have put the back on the watch securely and the o-ring is undamaged, the watch should retain its original water proof performance. The water resistance performance can be helped by putting a small smear of silicon grease on the o-ring, but be careful that this does not go inside the watch case.

Most high street jewellers will not be able to test the water proof performance of a watch on their premises, and so after changing the battery will not guarantee the water proof rating of the watch, therefore if you require your watch to be guaranteed water proof (such as a diving watch) it is likely that the watch will have to be sent away.
The test equipment required to do this costs a couple of hundred pounds and consists of a small tank which is half filled with water. The watch is suspended in the air space above the water and the tank is sealed. The tank is then pressurised with a hand pump to the required test pressure. Once at this pressure, the watch is lowered into the water and then the pressure released from the tank. If bubbles are seen coming OUT of the watch then clearly it is not water tight (well, not air tight actually). The seal will have to be changed again, the back retightened and the test repeated.

Water proof rating on watches can be confusing, see the Casio water proof rating chart for a detailed explanation of water proof rating on watches.