Have a look in HELP.
You will find under CLEAR Commands a reference to CLEAR CLASS Classname
Because the Class is held in memory you can not modify it until it is CLEARed

>>>Frequently after running a form, when I go to the pj to modify one
>>>of the classes used in the form (or the form class itself) I get the
>>>error "cannot modify class in use" (syntax may be wrong). I spent
>>>many hours trying to figure out where it was in use, but then
>>>discovered that I can modify a sub-class of the forbidden class, then
>>>then forbidden class can be modified.
>>>
>>>Example:
>>>
>>>run client.spx
>>>modify class frmBase --- error
>>>modify class frmData --- subclass of frmBase used by client.spx
>>>close frmData
>>>modify class frmBase --- works fine now
>>>
>>>What am I missing?
>>>
>>>TIA
>>
>>Sounds like you have an object based on the class still residing in memory.
>>When I get this error, I issue a 'clear all' in the command window.
>>I have found this clears remaining object and allows me to modify my class.
>>
>>- Chris Holtz
>
>Bill and Chris:
>
>Sometimes even a Clear All doesn't seem to work. In that case, I just close VFP and re-open it. Faster than trying to find what was left open.
>
>Barbara