Why can't Apple let us upgrade SD to HD?

just looking through store , @ least 4 movies bought when available in sd available in hd. why can't apple let upgrade small fee? getting point used hd , sd on big screen starting drive me nuts, , tv 42" don't see how people bigger tvs can take it.

 

it's not matter of ability, of choice. don't let you, it's not aren't able.

 

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