Hi Padraig,

It all depends on the specific terms and conditions of your agreement, but for the most part, it is possible to upgrade at nearly any stage during the term. Now, that doesn't mean it's cost-free. If the value of your existing car is less than the lender's calculated settlement fee, you'll need to make up the difference before you even think about your upgrade. Conversely, if the car is worth more, you'll have more money to use as a deposit on the next car. 

Now, all of this is likely to be smoother if you stick with the same car brand, so the first thing to do is go and speak with them and see what your options are. But saying that, there's no reason to stay with the same dealer (within a given brand) or even to stay with the same brand, though to change this takes a little more effort on your behalf.

Whichever way you want to do it, be up front with the dealers and you're likely to find them helpful.