You can remove the thumb turn by taking out the cotter pin but it won't give you access for a key. I don't know why you would want to though. Having a thumb turn is much safer in the event of fire or other emergency, you don't want to be looking for your keys at such a time. If you have to change to a key access inside, it is far easier just to change the Euro cylinder (lock barrel) usually around £5 to £15 and upwards, depending on make and type of cylinder you buy. This can be done in minutes but it mean having new keys.
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