If the values of the tags that you mapped for your TwinCAT HMI program is not updating anymore after a PLC program change, it is most likely naming or the path has changed for those variables. 

Go to TwinCAT HMI, then Windows and select TwinCAT HMI Configuration. 

Double click the 'Mapped Symbols' on the left menu, select the tags that are not updating or you can select them all, right click and select 'Refresh Mapping'. This will update the mappings for the symbols. Build your application and publish to HMI Server again.