Well, I can answer my own question now. The chain on the FE570S as-delivered from the factory is one link too short to go from the as-delivered 42 tooth rear sprocket to a 48 tooth sprocket. (That's without changing the 14 tooth front sprocket.) One link. Nuts! See photos, and note that with the chain adjustment blocks all the way forward, the hole for the axle is blocked.
Both the owner's manual and the spec sheet describe the chain so: 5/8 x 1/4". I sure hope that's a 520 chain, because that's what I bought!