In general,当 have two usages(and I think the understand you provided is right and integrity,I just need to give you some example) :)
1.## become, be ##
我当医生了。 Here,当is represent become, It is the present tense(the tense is related to '了',have nothing to do with 当)
2## would become, would be ##
有一天我会当医生. Cause of '有一天', 当 at here means would become,that is to say,he is not a doctor now,it is a wish,a hope.
3.## when ##
当天晴的时候我就去散步。 I will go for a walk when it is sunny.Here,当means when ,it is a time prep.
4.## face,confront ##
你不该当着他的面责备他。 You should't blame him in front of him. 当 at here is also a prep.
For the question you asked'How probable does the author think the noun after 当 occur?' 有一天我会当医生。(I will become a doctor one day), the noun(医生or any other jobs) would happen nothing,it is the wish the speaker have, whether his dream could come true is depend on himself.