In Designer 2.8, we added a new documentation record for Project Options (see File->Project Options).
Whenever you read a project from Disk with 2.8, if that project is missing that record, that new record type gets created. Hence, your 2.7 project file on disk that is missing that record, so when it is read from disk, that new record is created, causing it to "change".
However, your PLC project does NOT have that new record type. So when you save the PLC project to disk, it still does NOT have that record type. So, the next time you read that file from disk, it creates the new record type, "changing" it yet again.
To "sync" up the two, you must write the 2.8 disk project to the PLC. That way, BOTH will contain the new record type.