I'm guessing that the user will be prompted to pick a specific week when the report is run. Do you want 8 weeks prior to the user selected week or if the user selects a specific week in history, do you want 8 weeks forward? Is the WeekID sequential? Will the Week Attribute be on the report template? Here is how it could work out if your weekid is sequential. If the user wants 8 previous weeks from the select week, then create 8 filters following this logic. For the previous week from the select week:
Week 1 metric = Count(Customer) where {[Week]@ID -1}
Do -2, -3, -4 etc to get prior 8 weeks. For 8 weeks forward, user '+'.
This will give you customer counts for the 8 weeks prior or forward. Now you can to Current Metric - Week 1 metric to get the differenc2.
If the WeekID is not sequential, then there still should be a week start or end date column in the DB in date format. If so use Count(Customer) where {[Week]@StartDate -7}. -14, -21, -28 etc to get prior weeks.