Hi Cliff,
Yes, you can convert anything to an HD frame resolution (even VHS or Video 8), but it is not actually true HD. A single frame of true HD material contains 2.073,600 separate original pixels, an SD frame contains 414,720. What the up-converters do is try and make a best guess as to what those missing 1,658,880 pixels might have been, but as they never existed, the up-resing is just a very large amount of incredibly clever algorithm generated computerised guesswork.
For the broadcasters, there is very little point in up-resing archive material unless it is on a format which can have a reasonably acceptable HD master made of it, which rules out anything recorded to analogue tape formats. If however, you have access to something on film, then the results, even from 16mm, can be very good and are definable as HD. Even if a broadcaster wanted to up-res all their old analogue tape based material, the cost of doing all this up-resing would be immense, especially if your were recording it all to tape. (an HDCamSR VTR costs in the region of £80K without any options like the up converter boards).
Also, If you're going to say that you are transmitting completely in HD, transmitting old episodes of up-resed SD tape based shows is basically a con. However, all the main broadcasters like the BBC, ITV and Channel 4, Sky and in an increasing amount, CH5, all now require that new programmes are produced from start to finish in HD, so they are working towards a fully HD environment. Where there is a need for up-resed SD in a programme, (tape archive material in a documentary for instance) then compliance allows a maximum of 25% of the show to be of up-resed content but no more.
The infrastructure costs of moving to HD transmission is also very high. Every piece of equipment needs replacing, and most broadcasters would probably have started their HD infrastructure from scratch. Transmission costs are not something I could tell you about but obviously, as your signal requires more of that valuable bandwidth than for SD, the facilities providing the transmission route will be charging you a premium rate.
So for the major broadcasters there will come a time quite soon when their main channels are mostly true HD originated footage. From next year, BBC1, ITV1&2 will be simulcasting HD and SD, Sky transmits most if not all, of its HD channels as simulcasts - it is far easier and gives much better results downconverting the HD output into SD for the transmission routes. Classics and older material will stay how it is on SD channels - as the material in relation to HD material is of a much lower quality, these channels will rely on the in built up-resing technology in your TV to fill your HD screen.
And finally a general point about 1080i and 1080p which seems to be misunderstood by a lot of people. Namely that both these formats are full HD. The main reason for 1080 i being used by the broadcasters is that it is easier to transmit - bandwidth restrictions mean that it is easier to send 50 smaller packets per second than 25 ones twice the size. Very simply, all an interlaced signal means to film based material is that when the original film is scanned in either a telecine or a film scanner it is buffered into a frame store. When it is recorded onto the master tape the output of the telecine/frame scanner separates it into 2 fields, the first field being the odd lines 1 - 1079, and the second field even lines 2 - 1080. When these two separate fields arrive at your TV, they are once again buffered and re-interlaced as a single progressive frame just as the original film was. In fact the most common recording format for progressive based material devised by the wizards at Sony is 1080 PsF which stands for Progressive segmented frame, which in essence is just another way of recording a single frame in 2 parts - just like the fields in 1080i.
Hope there's something of interest in there.
Rob