Yes cable is not an issue but the fact how cables are laid is. there should be no noise or any interference in signal for an ideal digital reception. or even analog. if you are getting you analog channels from start till end clean and clear then there are good chances of getting digital channels. for digital channels cable operators use certain number of analog frequencies to stream digital channels. one analog frequency can hold as many digital channels as bandwidth can handle or depending on compression. cable operators usually compress channels so badly to squeez as many channels per frequency as possible. so when they say they are providing larger number of channels as compare to any cable network it means most of the channels will look crap on tv specially on lcds. regardless of HD or not. Reason for that is a limited amount of analog frequencies allowed by PEMRA to utilize for digital transmission.
As far as transmitting HD channels is concerned, its not a big deal now days. it all depends on compression. DWN is providing 2 full HD 1080i channels with good picture quality. Nayatel is providing 15 HD channels which i haven't seen yet so i can't comment on that. what farhan said is also true. watching crappy cable tv channels for years even a clear good quality SD digital channel will look HD to many people but one with proper knowledge can spot the difference on HD lcd screens. so you can ask cable wala to come at your house with digital box and check if you are getting proper signal and if only if there is a good quality signal then you can check there HD channels line-up on your lcd. thats the proper way.