Q) I keep it in a plastic container with a good sealed lid, its a white 5 gal bucket I got at Home Depot. Does wheat change after it has been stored for a few years?
A) ....Yes and No.... It all depends upon storage.
If stored properly grain is good for up to 15-20 years.
Properly storage would be in an air-tight container (the Home Depot buckets are secure, but not air-tight) with an oxygen absorber (actually 2 - 1000cc ones for a 5 gallon air-tight pail). In dry, cool (lower than 80 degrees) temps away from direct sunlight.
Another option is to vacuum seal grain in light-blocking mylar bags with oxygen absorbers and again keep them in a dry, cool place away from direct sunlight.
These are the correct long-term storing methods.
ANY deviation in even one area of either method will cause grain to begin to decompose, dry out (wheat germ oil on the inside begins to either go rancid or become more dense/dry/sticky) and the texture and taste will be compromised. It won't kill you, but it's not going to taste and act the same as well stored wheat.
Grain stored by any method not described above will have a MAXIMUM fresh life of about 6 months to 1 year. So, your grain from 2009 was pretty over the limit given (potentially) the conditions and time.