Maybe I put more into the flexibility of the thumb, I'm old, thumb just doesn't bend the way it did years ago, but for me left is a must. I've changed a few customers to more left and they say it 'feels more natural', but again, the flexibility is the biggie.
Over the past year also had more go to more reverse in the pitch ( under ) and they say, they feel better. I hate to make just one change, but it ALL depends on the thumb/fingers flexibility, swelling or not and skin.
Good Luck on the changes. IMO, you have to make a change every now and then. Your joints change, swelling/no swelling happens and if you don't keep up with the changes you'll hurt in the end.
JMO
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az guy aka: R & L Bowlers Pro
rlbowlerspro@cox.net
www.rlbowlerspro.com