What do you mean by import?
It is my understanding that a subreport is part of the main report.
if it originally was its own report and you use the 'Choose an existing report' when inserting a sub-report it makes a new version of this report as a subreport.Changing the orginal does no impact the inserted version.
So if you have:
Report A
Report B
Report C (used as a subreport via insert subreport, choose and existing report into reports A and B)
Making a change to C does NOT change the subreports in A or B.
Making a change to A's subreport does NOT impact B's subreport or C (and the same applys to altering B's subreport).
If you want to make a change you have to either
Change the subreport in Report A and then also make the same changes in Report B's subreport
or
change C, save it then open Report A, delete the subreport in it then use the insert subreport and reinsert it using the 'Choose an existing Report'.
repeat the process for Report B.