Is there some simple express way of doing this without saving a copy then reinserting and remating again.
Use Configurations in the part.
2014 SOLIDWORKS Help - Configurations
Create a second configuration of the Part, to the desired dimensions, then reference this new configuration for the second instance.
If you only have one model and it doesn't have configurations, you will not be able to show the same model in different sizes. Does the part have configurations representing the different sizes? If you need a different model file, you can save a copy, make changes and then use the "Replace" command to just swap it out, no re-mating required.
you could use configurations.. at the assembly level right click on the component & choose "add configuration" type a name & then click OK. at this point solidworks should create a config in the part & switch that 1 instance to that new config. Then just change the part (make sure you choose "this configuration" when changing dimensions)..
You could also make both of your instances virtual parts and then make them independent from one another. Then you can save them with different names for each of the parts. After you can make the changes to the one you need to, without affecting the other one.
If you have 2015, you can make one of the instances of the component unique by right-clicking on it and selecting 'make independent' from the context menu.
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