This may be a stupid question but I was wondering if a dog's hip scores can change.  For instance, if a dog has a good hip score at 1 year and is allowed to become overweight etc and then has another x-ray at 3 years would the score be worse?  Also, if a dog had a poor hip score and then had hydrotherapy etc would their hip score improve?

Yes and no. You can only have a dog's hips scored ONCE in a lifetime, so you will never get to know a different score. (Not unless you object to the original one and is granted a review of it, but this is pretty rare.) However yes, hips can get worse as a dog ages, but that is usually a case of the hips having been pretty bad to start with, and then getting worse as the dog gets older -it's not particularly likely to be different between the ages of one and three. Maybe between one and seven.

Less than perfect alignment for x-ray however, can have an affect on scores.  Good alignment wont make poor hips look good, but poor alignment can make good hips come out with a lower score than someone who aligns the dog well.  So if you were able to get the hips scored on a regular basis, there may well be different numbers being produced dependant upon the skill of the person carrying out the x-ray!

Excellent article about this at http://leerburg.com/hipart.htm

Not sure that something like hydrotherapy would improve a score result ... the scores are looking at the structure of the hip joint. Whilst hydrotherapy certainly helps dogs with hip problems, that's something that's building the muscle around the joint rather than re-defining the actual bone.