An e-mail from a reader arrived this morning, asking what had happened to ''Threshold'' on Fridays. Since some of you may also be wondering, it's a CBS shuffle to try out both shows in different time slots. ''Close to Home'' switched to Fridays at 9 p.m. (the ''Threshold'' slot) last night and on Nov. 18. ''Threshold'' will be on Tuesdays at 10 p.m. (the ''Close to Home'' slot) on Nov. 22 and 29. (On this coming Tuesday, Nov. 15, CBS will have the CMA awards.