The conversion coming really doesn't have anything to do with HD. What they are doing is getting rid of analog (rabbit ears and roof atena's) and everything is going digital. So really only impacts people that have the old rabbit ears instead of digital cable or set top box. So this will have not have the impact your thinking of actually this conversion shouldn't effect most people at all.

Just the die hards that don't want to order cable/fiber/satelite or upgrade there old TV's.

I would imagine that the HD on 1 TV has something to do with bandwidth over
copper. I think it would be safe to say eventually they will have multiple HD
streams to compete but market wise most customer don’t need more than one HD
stream beyond the handful of households that have multiple HD Tv's. 

If I were to make a logical guess I would think the only upgrade fee you
would have to make is if you needed an HD box versus a standard box when.

 Another educated guess would be that once the demand is there and it
clearly financially beneficial for the company is that they will extend the
fiber line all the way to the premise (to the side of the house) and eliminate
the user of copper all together. At that point bandwidth would not be an issue
at all and exceed any other current technology.

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Please note the above information is based on opinion
only.

Hope that helps!