When you substitute a proper noun, replacing it with an ordinary noun, does that ordinary noun become a proper noun and require capitalization?
In usual practice, don't capitalize the common noun. However, if you want to achieve the specific effect of highlighting the proper noun you are referring to, go ahead and capitalize.
I think the question is asking that if we identify a proper noun in an initial sentence and then we refer back to that same noun, perhaps in a following sentence, using a common noun, do we capitalize? Let's see:
I used to live on the north side of the Golden Gate Bridge. Every day when I'd commute into work, a paint crew was always already hard at work, protecting that immense structure from the effects of the wind and salt. That paint crew, by the way, has only one job, to wit, to continuously paint the bridge from one end to the other - and when they get to the end, they start anew at the other end. It's through this constant state of maintenance that this symbol of California and of San Francisco is able to age as well as it does.
For the most part, when using a common noun, even when referring to a proper noun, we don't capitalize.
However (yeah... there's always exceptions, aren't there...), there are times when we use a common noun that is referring to a proper noun and we want the reader to know that we're doing it, that we can capitalize a common noun. Sometimes when we're doing this, we haven't identified the proper noun, but only made a vague reference to it. For instance, I can write:
I recently returned from a trip to the San Francisco area and I must say that I was most impressed by that Great Engineering Marvel that spans the channel between that City and it's neighbours to the north.
Here, I've played with capitalization a little. I wanted to refer to the Golden Gate Bridge without actually naming it - and to highlight what I was doing, I added capitalization. Also, I wanted to highlight my use of the word "city" and so I used "that City" - both the use of "that" and the capitalization do that. I could have said "the city" and that usage would have diminished my reference to the city. If I don't capitalize any of it:
I recently returned from a trip to the San Francisco area and I must say that I was most impressed by that great engineering marvel that spans the channel between the city and it's neighbours to the north.
Feel the difference? All the highlighting is gone - the bridge becomes much more humdrum as does the city.
And so, in usual practice, don't capitalize the common noun. However, if you want to achieve the specific effect of highlighting the proper noun you are referring to, go ahead and capitalize.