Just migrated our entire RUM setup from Tool X to PerfBeacon last week. Took 4 days, start to finish, with zero data gaps. The team thought I was crazy aiming for that, but honestly, if you're staring down a multi-week or month-long migration plan, you're probably building a Rube Goldberg machine.
The secret? Don't try to migrate *data*. Migrate the *instrumentation*. Your old historical data stays put in the old tool for reference. Your job is to cut over the data *collection* as cleanly as possible. We ran both scripts in parallel for 72 hours to validate metrics matched, then killed the old one.
Here's the core switch. We replaced this legacy snippet:
```html
```
With the new one, but wrapped in a little feature flag for sanity:
```javascript
// Load new RUM
import { initPerfBeacon } from '@perfbeacon/rum';
if (window.FEATURE_FLAGS?.NEW_RUM_ENABLED) {
initPerfBeacon({
apiKey: 'new_key',
sampleRate: 1,
// Capture Web Vitals immediately
webVitals: { reportAllChanges: true }
});
}
// Legacy tool loaded conditionally elsewhere for the overlap period
```
The parallel run was crucial. We compared CLS, LCP, and FID percentiles between the two datasets daily. Seeing a <5% variance gave us the confidence to flip the flag globally and remove the old script. The actual "cutover" was a one-line config change in our deployment system.
Biggest time sink? Not the tech, but deciding on the exact set of metrics and thresholds we wanted to alert on in the new system. Should have locked that down *before* we wrote the first line of migration code. So, what's the longest part of your migration? I'm betting it's decision paralysis, not the actual execution.
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