Heidiho,
If your temps don't get down in the 30's, you won't ahve any problems with your hibiscus over the winter.
I haev one in the ground that recovered nicely and we get down in the mid-20's here on occassion.
As far as repotting goes, you could do it now. Just make sure you don't repot into a much larger pot. One that's one size up (2" wider and deeper) is good especially this time of year when the plants are slowing down their growth rate. Hibiscus hate wet feet, so potting in too big of a pot isn't a good idea - the roots can't reach enough of the dirt, stay wet and you run the risk of root rot.
To keep them in tree form, yes you just keep trimming off the lower branches.
Barb