Policies vary widely by Comcast area, but techs have longer hours these days, we are lucky to have them 7 days a week until 7pm. You might be able to get a late appointment.
Unless you have the skills to troubleshoot your cable wiring inside your house, a tech visit would be best. What might have happened is as the channels increase, Comcast is using higher frequencies to carry all those new HD's. Higher frequencies = more loss on the coaxial system, and this affects, the loss in the coax, splitters and F connectors. Many homes that have had cable for many years have splitters not designed for the higher frequencies, now splitters must be rated 5-1000mhz, where the old days, splitters only went to 500-600-750mhz. These old splitters, old cabling, and old crimp F connectors affect these new channels on the higher frequencies.
Without the proper test equipment, you don't know how the signal is at your outlet, at all across the frequency band. It is very common to have issues with only part of the frequency band, knocking out or supplying low signal levels at only some channels. This causes the tiling only on some of the channels. Sometimes it's poor / wet / corrorded F connectors, squirrels chewing on the drop cable, or old splitters.
It's your schedule and decision, but a tech visit would be best. If want to attempt a do-it-yourself solution, post back.
As far as the upgrades, no one here would know the schedule, if you could ever stop by your local office, they would know more until they start sending you mailings about the digital transition.
