I'm afraid that there is no elegant way to resolve this issue.
You will not be able to delete the security while it is in use. e.g. the security is used in transactions and / or reminders.
Something that you may be able to do to make the process a little easier is to rename the 'old' security to something that can be easily seen in the register. e.g. rename the security to 'DELETE THIS SECURITY'. Then create a 'new' instance of the security in question (with the desired decimal places) and mimic the 'new' securities transactions in the register with the old security. You can then, one-by-one delete the 'old' security transactions and eventually delete the 'old' security altogether.
Safe computing 101 dictates that you make a copy or make sure to have a working backup of your data file before making any drastic changes (especially deletions).