Downloading a config from different platform is possible. It is supposed to prevent any config statements that are not compatible with the target platform. I'd save a clean config from the new switch, then download to the 6510 the config saved from 5100, and then again upload the config from 6510 for a comparison with the "clean" previously saved file. Diff / sdiff are usually helpful here. If there are any unclear changes, it's better either investigate them or revert to the stock config and replay any required changes manually. BTW, do you have many of them in your setup? Of course, all of this is possible to do in advance, while your old switch is still in production.