If you upgraded to SCOM SP1 from SCOM RTM you will notice that you are no longer able to edit your web monitors that you created before the upgrade. Any Web monitors that were created after the upgrade, you will be able to edit fine.
The error you will get is “There was an error while applying the changes.” “An object of type UnitMonitorType with Id (GUID) was not found.
If you are having this same issue a fix has been created by Microsoft. You will have to call Microsoft Support to the the hot fix. The KB number is 951527.
This fix is not publicly available through the Microsoft website as it has not gone through full Microsoft regression testing.