If all of the tubes - new & old are all matched / all rated the same you might not need to bias. The new and old all have to be the same / identical match.
Bias drifts .... If I bias a amp a week or less later it usually needs to be done again. As tubes are used the bias changes ... so if they start as a matched set - soon they are not often.
I bias every-time I change tubes and check them from time to time.