No. Certain technologies can only be put into place in new homes. But insulation, and weather sealing can be added at any time - along with better ventilation. If the homes are built to modern standards, the windows will wear out in fifteen years at most, and can be replaced with more efficient ones. If you are lucky enough to have older sturdier windows that last forever, you can still retrofit them with window boxes, insulated curtains and other types of additions.
Look, I'll agree with you that suburbs are not inherently bad - there stuff that is desirable in both types of living. I'll add cities tend to use resources more efficiently than suburbs - but the difference is not as great as people are making it. We need to make both cities and suburbs more efficient; neither is at a sustainable resource usage level currently.
But in the U.S. the dominance of suburbs over cities, and the dominance of automobiles over trains has little to do with peoples individual desires. We had a working light rail system in the U.S. that was destroyed, the tracks torn up the the cars sent to crusher, and replaced with a truly horrible bus system. Suburbs are subsidized by cities in very real ways - road building, water, sewers. Also the suburbs were largely financed by the GI bill - and that money was quite deliberately directed to suburbs in part to avoid paying in African-American arears to maintain segretion. In short, like the automobile, the suburbs were shoved down our throat. If you subsidize something heavily enough, you get more of it. We subsidized suburbs at the expense of cities, automobiles at the expense of trains. Naturally we ended with balace tilted unreasonably towards suburbs, and away from cities, towards automobiles and away from trains.
I'll talk from personal experience. I've lived in a reasonably dense suburb that mixed lots of parks and green areas, woods you could get lost in and also with shops, decent libraries, an independent book store or two, small colleges, and even a little comnmunty theartres; it was very pleasant places indeed. I've also lived in a scattered spread out suburb where you have to drive miles in one direction to the grocery store and miles in the opposite direction for dry cleaning. I have to admit this really did strike me as a foretaste of hell, though I'm sure there were people there who liked it, and people who have found my "pleasant" suburb just as hellish.