I hope I'm wrong but I think I've found a glitch in the way VFp appears to clear a class definition from memory. If no instances of a class exist and SET CLASSLIB TO is issued then all the class definitions should also be released. However this doesn't appear to be true...

Here's an app that demonstrates the problem (you'll need to build a project t1 as described below before running this)...

* Here's the code from t1.app's main program...
*	set classlib to tshell
*	local x
*	x= CreateObject('MyClass')
*	* this class should be created visually and added to t1 
*         project - simply a generic "custom" class

* === Test Program Start ===
cs= 't1'
build app (m.cS) from (m.cS)

do (m.cs + '.app')
set classlib to

ok= .T.
on error ok = .F.
x= CreateObject('MyClass')
if ! m.ok
	* this is what we expect of course
	MessageBox('Cannot create a "MyClass" object', 64, 'TShell')
	* so it looks as if the class library has been properly cleared
endif

ok= .T.
erase t1.app
* will fail
messagebox('Erase before clear class: ' + iif(m.ok, 'Succeeded', 'Failed'), 64, 'TShell')

* This shouldn't be necessary, but is!
clear class myclass
ok= .T.
* will succeed
erase t1.app
messagebox('Erase after clear class: ' + iif(m.ok, 'Succeeded', 'Failed'), 64, 'TShell')

on error

The problem this creates is that I can't easily run a 'shell' program which self installs new versions of a program because any module which does get released properly remains bound up in memory. So even though I can actually fire up the new app the old app remains 'somewhere in the ether'. I would imagine that this issue would have implications for things like COM objects as well but I've yet to play with those.
I can work around this by explicitly clearing every single class. But I know of no command/function that will list the classes that have been instantiated at some point.