Is this change normal? I have recently had my tyres on my bike changed from mountain bike tyres to road tyres. I was also advised by the man who fixed it that my saddle was too low for me and he raised it. Since these changes it almost feels like I am cycling with slightly flat tyres. Uphill doesn't feel massively strenuous but I just wanted to see if this is normal or not?
Re: your saddle being too low. 

One way to check if it's at the correct height is to sit on your seat, put one foot on the pedal and try to "straighten" your leg - with the pedal at it's lowest point (the other being at the top) there should be a slight bend to your leg.  

This video's quite informative: seat height