Change of custom field type to bigint
You have all the control of your custom table but always it is not advised to change custom field type in the database 

Better way of doing this would be

In DB - Create a temp table of the same and move all the records to the temp table first

In the front end - delete the column with varchar and create a new one with int with the same name 

In DB - move all the value from temp table to the custom field 

I guess this helps !!!