When you make changes to your script, you make the changes on the disk(hard disk- the permanent storage); when you execute the script, the script is loaded to your memory(RAM).
So, the changes that you make to the script will not affect the running script, it will run the version you executed before making those changes.
However, when you execute the changed script again without terminating the previously running instance, there will be two instances of the script- one which has the changes and the old one.
Be warned that the resources that the script uses and modifies will conflict. For example, if you are modifying a file using the script, the script that runs later will not be able to open that file for writing and fail to execute correctly.
Thanks to Registered User for pointing me to a better answer on Unix.stackexchange.com.
Depending upon the size of the script and the compiler/interpreter in question, the script is loaded partially/completely. So, if the script is not completely loaded, the changes that you make to your script will reflect on the running instance once the part of the script is loaded into memory.
So, it is not recommended to change your script on the disk which is currently running for unpredictable output: First stop the running instance and then modify your script and then re-execute the script.