Drop constraints vs disable constraints during a data import
As recommended by Mikael Eriksson, I'd do the import without dropping/disabling the Foreign Keys.

The downside to this is if you want to delete any records who's values are referenced by an FK or want to update values such that the original value is no longer in the table and referenced by an FK.
In these cases you'd need to keep track of any such issues and decide how to handle each "conflict" (don't update/delete, alter the data in the other table or some other solution).

This will be quicker, easier and ensure data integrity compared to dropping/disabling the FKs, importing and then restoring the FKs.