The size of the area that this planting grows in would be helpful in determining whether to temporarily replant it in the ground or pots. From what you say it seems happy where it is so you will want it back in the same place. Also, how long will it take to replace the wall and also, if a cement foundation used and mortar to join the bricks the surrounding earth might become very acidic for a while. A photo of this will be very helpful.

The old wall is probably held together with mortar which has weathered and is soft so that the roots of the campanula can penetrate it. The new work will have fresh mortar which is caustic and will set hard so you cannot plant into it for several years. Can you wait until the campanula has set seed? If so, then collect some seeds and sow in a pan before removing the parent plant. Plant the campanula in a pot with a gritty compost and keep it for a couple of years. Get the builder to leave some vertical spaces in the wall by not cementing between all the bricks; maybe he could even leave the occaisional half-brick out so that you could fill the gaps with some compost and replant at a later stage.

Will the new wall have cracks? Just wondering how you plan to install the campanula to the new wall. You can affix planters to the new wall which could work. Planters can even be built into the wall if you have a skilled mason or just leave a few gaps in the mortar where you can simply plug in the campanula. There are a few ways to approach this but resolve this question BEFORE you begin construction.