I have an exsiting field that was built as a text box.  We are having some data entry spelling problems so I want to change this text box to a drop down box.  This field currently has data in it and I don't want to lose that information, but I want to add more information to the drop down list.  Is this possible to do?

I am still new to salesforce and I am getting the following error on my education hours object.  I don't know where to start looking for the problem.  Can somebody give me guidance on how to solve this issue.  Here is the error message: 

dlrs_c4g_Education_HoursTrigger: execution of AfterUpdate caused by: System.SObjectException: Invalid field c4g_Class_Hours__c for c4g_Education_Hours__c Class.dlrs.RollupService.handleRollups: line 817, column 1 Class.dlrs.RollupService.triggerHandler: line 311, column 1 Trigger.dlrs_c4g_Education_HoursTrigger: line 7, column 1

I am still new to salesforce and I am getting the following error on my education hours object.  I don't know where to start looking for the problem.  Can somebody give me guidance on how to solve this issue.  Here is the error message: 

dlrs_c4g_Education_HoursTrigger: execution of AfterUpdate caused by: System.SObjectException: Invalid field c4g_Class_Hours__c for c4g_Education_Hours__c Class.dlrs.RollupService.handleRollups: line 817, column 1 Class.dlrs.RollupService.triggerHandler: line 311, column 1 Trigger.dlrs_c4g_Education_HoursTrigger: line 7, column 1