Yes, concatenated just means that the put your X results after the autosomal in the same file. Currently, they still say use Build 37. (Build 38 exists, so presumably at some point they will switch to that.) If you are worried about overwriting, you can upload the files as separate DNA kits that will be assigned their own IDs. But as far as I know, there is no reason to want just the autosomal file uploaded, so overwriting it should be a good thing.