i have one of these and I just prune it lightly. It always does very well.

https://www.rhs.org.uk/advice/profile?PID=346

Have a look at this link Nellie. If you need to repot you can do it when dormant.

Do make sure that the pot and base of the plant are shaded. Clematis require this for good growth.

I don't know who told you to cut it down, but the usual reason for doing that is to encourage more shoots off the roots, and to make it easier to handle the plant when potting on. Although the pruning recommendation for this variety is basically just to tidy it up, it will come to no harm if you cut it right down, it will regrow, and if you'd not had all those flowers this year, then I'd agree to cut it right down.The trouble is, The President is one of those clematis that flowers twice - the first lot of flowers are large and fewer, then about six weeks later, another lot arrives, this time smaller and more plentiful. If you decide to cut it right down, you can do that in winter, by end of February, but you will lose the early, larger flowers, though will still get the later, smaller ones.

If its performed well this year, then just repot into something larger - this can be done in early spring if its easier (the topgrowth will likely look quite dead by then anyway, although if you want the earlier flowers, try not to break too much of it as you repot).