The nvl() function is just going to replace No_value/ Null with 0.
So, you are asking if you add aggregation function (sum, count, etc.,) in this measure does it affect the existing reports,
Can't say exact answer, because
Before : your backend sql,
select column1, column2, column_name, ... from table_name
Result set : 100 records
After you add sum(): your sql,
select column1, column2, sum(column_name), ... from table_name
Result set: 2 Records
This sum() is going to do a group by () on your database and bring the aggregated values and improves performance and may drastically minimize number of records. So if the report developers puposefully didn't put sum() in this measure (Ex: They want see each record without any aggregation), then adding sum() may affect that report,
If this Measure is just used to find totals etc, then its not going to affect the reports
For more info: SQL Aggregate Versus Universe Projection