Summer reruns never seem to hurt the most popular prime time sitcoms--their ratings may take a dip during the warm weather months but bounce back when the new season starts. I don't see why Jeopardy! should be any different.
The biggest concern I can see is that Ken's first day back happens to be Sept. 6--Labor Day. Unless they're planning another rerun for that day and Ken won't be back till Tuesday, I fear a great deal of viewers will either miss it because they're too busy traveling or barbecuing in the back yard, or because Jerry Lewis will be seen during that half hour on certain local NBC, CBS or ABC affiliates (NOT the entire network, of course) chatting with EdMcMahon, introducing an old showbiz crony or yelling, "Drum Roll" as a camera is pointed toward an MDA tote board whose numbers take their umpteenth climb upward.
If Ken just happens to take a fall that day, those stations with commitments to both J! and the telethon are going to be swamped with calls demanding a make-good rebroadcast. I don't think it'll be a problem in Omaha or in Kansas City, but it could elsewhere.
The CBS debuts of The Joker's Wild, TnPIR and Gambit went unseen in the two largest Iowa markets for the same reason, with only "Joker" continuing to regularly suffer the pain of pre-emption in Des Moines after everyone went back to work the next day.