Lesson 4 of 5
Run In Parallel, Then Cut Over
The dangerous moment is the switch. If you publish GTM while the old tags are still hard-coded, everything fires twice. If you rip out the old tags before GTM works, you lose data. The answer is a short, controlled parallel run, then a clean cutover.
1. Old tags livebaseline→2. GTM in paralleldebug only→3. Verify matchside by side→4. Remove oldGTM only
The safe sequence
- Keep the old tags live. Build GTM and verify it in Preview against the same events.
- Compare GTM's hits to the old tags' hits in DevTools, same events, same values.
- In one deploy, publish GTM and remove the hard-coded tags together. This is the only moment that avoids both gaps and doubles.
- For analytics where a brief overlap is harmless, a short double-count window is acceptable; for ad conversions it is not, coordinate the swap tightly.
Key takeaway
Build and verify GTM while the old tags still run, then publish GTM and delete the old tags in a single coordinated change. That one move is what prevents both data loss and double-counting.