Your pump won't have a problem, BUT your wire will, the reason you put in a heavier guage wire over longer runs or for higher loads is because the 14 guage will heat up, won't be an issue the first year or two or three, but the insulation will become brittle due to the heat that's created in the wire - less heat will be created in the 12 guage in comparison and the insulation will therefore last way longer.
Brittle insulation will lead to a short, a short can lead to a fire, a fire can lead to a homeless family or God forbid, something even worse.
Don't mess with your families safety!

Oh, BTW - if the existing wire is 7 years old allready and was possibly overloaded, it could already be brittle or very close to being a hazzard.