You’ve been running successful Google Ads campaigns optimized with Universal Analytics goals. After migrating to GA4, you switch conversion tracking to GA4-imported conversions. Campaign performance drops for 2-4 weeks. Bids go haywire. ROAS collapses. This “migration dip” happens because Smart Bidding’s model needs to relearn conversion patterns with the new GA4 signals. This guide explains what causes the dip and how to migrate without destroying performance.
Why Smart Bidding Needs Time to Relearn
Smart Bidding (Target ROAS, Target CPA) builds a model from weeks of conversion data. When you switch conversion sources from UA goals to GA4 conversions, you’re giving Smart Bidding a new signal it hasn’t seen before. Even if the GA4 conversion represents the same user action as the UA goal, the signal is technically different:
- Different conversion count (GA4 may count conversions differently)
- Different attribution model (GA4 default is data-driven; UA was last-click)
- Different conversion window timing
- Historical data is GA4 data starting from when GA4 was implemented, not from campaign start
The Safe Migration Approach
Never switch abruptly. Use a parallel-running approach:
- Step 1: Import GA4 conversions into Google Ads but set them as “Secondary” (for observation only, not bidding)
- Step 2: Run GA4 and UA conversions simultaneously for 4-6 weeks. Monitor the ratio between them.
- Step 3: Once GA4 conversions have 3+ months of stable data in Google Ads, switch to Primary.
- Step 4: Set UA goals to Secondary (don’t delete yet).
- Step 5: After 2-3 weeks confirm Smart Bidding is performing as expected, then deprecate UA goals.
// Importing GA4 conversions to Google Ads: // 1. In GA4: Admin → Google Ads Links → Link to your Google Ads account // 2. In Google Ads: Tools → Conversions → Import from Google Analytics // 3. Select your GA4 conversions (purchase, lead_form, etc.) // 4. Set as "Secondary" initially // Monitoring the ratio: // Expected: GA4 conversions / UA conversions ≈ 0.7 to 1.3x // If ratio is > 2x or < 0.5x, investigate before switching