Hi Kalyan,
Any potential impact can only be realized after you run the report. I'm not too sure how this report has been designed however, if you interchange the Main Report and the Subreport I guess that would completely change the report layout (?).
If everything including the layout and data are correct then you have nothing to worry about. Otherwise, you would obviously need to use another subreport in the header.
Also, I read the post again and see that the Subreport has been placed on the Report Footer and linked on a column. In which case, the Subreport *will only receive the last value in that column* to be linked to its parameter.
Why not just create a formula (maybe Maximum({digit_column})) to get that last value and show it on the Report Header?
-Abhilash