When you switch to a model that’s backwards-looking (anything other than last click) it won’t track retroactive clicks as conversions. In a way, you’re starting from scratch.

If you typically need 1-week to attribute all your conversions you’ll have 1-week of down performance. You can get an idea of your typical attribution from those reports in Google Ads or Analytics.

Also, importantly, you need to use a longer look-back period to evaluate performance. If it takes a week for most of your conversions to be realized you shouldn’t be looking at the most recent 7 days of conversions.