Alex, you do have quite a quandry there on your hands. I would be tempted to try and move it even though it will set the plant back when you do so. It make take it a year or two to get back to its current state. I wouldn't worry about having to dig up the lawn and any residual nitrogen in the soil since the grass will more than likely have used up any previously applied fertilizer. Addition of more topsoil would help dilute any that is left.

If you weren't having to add the foot of soil all at once to the existing site you could probably keep the clematis where it is. We are constantly telling people that clematis should be planted deeper in the ground than they are in their original pots, but one foot of soil would be an awful lot to add at once over the crown of the plant.

If you could place a bottomless drum around the plant using say a cardboard form that is used to pour concrete or something similar, you could gradually add soil into the form over the course of a year or more as the clematis grows and that could possibly work. It would be definitely more work than just moving it but only you can decide which you are willing to try to keep it in its current location.

Let us know what you decide and how it works out.