Either the value has been set on load, in which case it's not significant, or it has been set through modifying other fields, in which case their "dirty" property will kick in.
So it's probably OK to set the saved initial value to the same as the "set" value./
We considered a number of different solutions for this issue.
Our guiding principle is that we want to allow the administrator to specify the default value for a field, and not keep the original one just because the user had edited it. Our other guiding principle is we don't want anyone to lose work.
We rejected solutions such as append the original text as we think that would cause confusion and end up with users submitting both.
We also rejected displaying an alert when they are about to lose the value, as it's not obvious what the user should do to not lose their text and switch project.
Instead, when a behaviour overwrites a field on project/issuetype change, the overwrite will happen but we will show a flag explaining what happened with the previous value(s), so the end-user can copy and paste whatever parts of it they want to retain.
Please let us know if that won't meet your needs.