For Example
1. The last customer number in the system is 5001
2. A new customer record is created and the user overrides the next number and instead assigns id = 5007 manually
3. Goes to Setup > Company > Auto-Generated Numbers, and applies the following changes;
Sets Prefix = X , Suffix = Y
4. Marks the Update box and clicks Save
All the existing customers will update to include a prefix of X and a suffix of Y on their numbers, but the customer record (id = 5007) created above which was out of sequence will now show the number X5002Y.
Solution
Users can enable Allow Override on auto-generated numbers (Setup > Company > Auto-Generated Numbers). With this setting enabled, users can override the system-generated number and instead use their preferred number.