change field from int to big int in drupal
Use the bigint module.    


  The default core fields do not include a bigint field. This module
  allows you to have a true BigInt (19) value. That means the number can
  contain 19 digits.


So, with this module you can define fields with the appropiate range.

Your problem now is to migrate data from your current field to a new bigint created field.

The usual steps will be:


Create and configure the new bigint field.
Edit affected nodes copying data form old int field to the new bigint field (using Batch API, for example). Or using a direct SQL commands, if your are brave enough.
Update related configuration to use the new field (for example, Views).
Delete de old field.


Depending on your usual workflow you can do this by different means:


Manually, closing the web during update.
Using Features for the configuration (fields, Views, etc).
Update script (for example, a hook_update_N if you are using a installation profile).