How to change existing design with minimal amount of work?

Question

I opened the schematics and layout of a design that was done four years ago. 
 I intend to make some changes. For a check, I synchronized the layout to the (old) schematics, saved the design under a different name and tried to synchronize again. To my surprise, there were a lot of parts that were pretended to have been removed and added. Where does this behaviour come from? Could a different path to parts - libraries be the cause for this? 
 
 Is there a defined, proposed way of doing changes to an existing design with the aim of minimizing the overall amount of work? Especially routed traces should be kept whenever possible, preferably even if there has been a change of the net name (while the RefDes of connected components stayed the same). 
 
 Kind regards, Manfred

RFinley · Answer

Some engineers replace passives in RF designs by hitting the delete key and plopping in a new part instead of linking the new part. Annotate. Done. Placement changes are a risk. Often, a part is deleted from one circuit block and new parts added elsewhere. Annotation uses previous designators. 
 You can always safely export your existing placement to IDF using the filter, import the netlist, import the IDF file and look for re-used designators not connecting to nearby C-lines. 
 I cross-probe verify placement groups before release. Especially for customer reference designs. Placement that doesn't follow the schematic. Ugh.

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How to change existing design with minimal amount of work?

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