This is just my opinion but I would not change to another drug based on that one sample since there is no evidence of hemorrhagic cystitis (straining to pee, blood, etc).  I also do not think you have any evidence for a UTI since that is a caught sample.  Cephalexin is a pretty benign antibiotic but why give it if it is not needed?

We are lucky to have an ultrasound at our practice so it makes it easy to see the bladder and get a sample.  I usually put dogs less than 150 pounds on their back in a padded tray to support their muscles.  Larger dogs or dogs with bad hips or that don't like to be on their back I will just do in a standing position.  There is also a blind technique that is pretty easy to do on most dogs that does not involve using the ultrasound.

So if I were in your position I would not give cephalexin, I would continue Cytoxan and piroxicam, and I would get a sterile urine with a culture to see if there is any evidence of UTI.

Every vet has their own comfort level with sample techniques, treatment plans, etc but you should not hesitate to state any concerns that you have.

Pam

By the way I have been to VA which is beautiful but I fear the heat would kill me!