Purchase for unknown domain

I am building a web site for a charity using Sitefinity.
They have currently a website live.
They are also in a bit of a pickle re domain name.
If all goes well they will get their domain back and we can purchase sitefinity, however they might not get it straight away, or might have to change it within the first few weeks.

I am not sure how we should go about this. I would like to purchase sitefinity right now so that i can guaranty the support as well as simply being able to run it live parallel to the original site for a few weeks. After which i am hoping to simple move the domain to this new sitefinty site. However in the unlikely event that they cannot get their original domain back they might have to go with a new one.

Is there a possibility to register one domain now and then change it later? If not what would you recommend to do in this case?

Dear Andrew

I am just running it in trial right now. Hopefully will have control of the domain soon, and it will all be moot.