USBC rule:
118b. How Established
Each league shall adopt a rule to determine the number of games required to establish an average in that league. The following applies when establishing an average:
1. A right-handed bowler must always bowl right-handed. Similarly, a left-handed bowler must always bowl left-handed.
Penalty: Forfeiture of game.
2. No combination of scores bowled both right- and left-handed can be used to compute an average, except as stipulated in Rule 4c.
3. The league’s board of directors may, by majority vote, adjust a player’s average before the player bowls in the league or during the season due to injury or disability. Separate averages must be maintained.
4.Adult Leagues:
The league’s board of directors may, with two-thirds vote, adjust a player’s average
upward during the season. Separate averages must be maintained. The bowler must use the adjusted average or established league average (whichever is higher) for the remainder of the season.
118e. Establish New Average due to Change in Delivery
If because of injury or disability a bowler finds it necessary to change delivery from right- to left- handed or vice-versa, the league board of directors, by majority vote, may permit the bowler to change delivery. If approval is obtained, the bowler must establish a new average.
Commonly Asked Question – Rule 118e.
118e/1
I hurt my bowling hand and was granted permission to use the other hand. Once my
bowling hand is better, what do I have to do before I can go back to using my regular
bowling hand?
You must get permission from the board to return to the original hand you started the
league with.