Join Date: Feb 2010
Location: Narragansett Bay
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Renumbering runways is very common. However, its doesn't happen all that often since runway numbers are the closest tenth of their magnetic heading. ie 340 degs = runway 34. That also means that 337 degs = runway 34. Variation usually needs many years to move enough to round differently, but it happen all the time.
How often variation will alter, somewhat depends on where you are on the planet. Suffice it to say, that it moves slowly enough that even a chart that was fairly outdated, wouldn't be off by more than a couple of degrees. For coastal cruising, this is a non-issue. Crossing oceans can certainly be an issue.
The deviation of your compass is probably a more significant issue than variation. Each aircraft that I fly must have a deviation card attached to the compass, and it can read off by 5 degrees on some headings. We can also cross variation lines often on a single flight. In smaller planes, you must continually reset your heading indicator to your magnetic compass (for other reasons too, which I won't bore you with). In the larger planes that I fly, this is all slaved and resets automatically.