The portal stops working and you keep the warp. You may contact a GA+ to remove the portal sign if you wish.
To answer the question for anyone who might come across this later, You are able to "trade-in" the portal for a warp credit if you're the official owner (not legal owner) If you're not the official owner, you also get the option of removing the portal, and getting $40k ECD towards a different town feature.
No. At no point are we giving Ecodollars. You may get a town feature or another warp. Anyway I am locking this thread as all portal -> warp questions should be looked for and posted in the sticky thread to avoid confusion. http://www.ecocitycraft.com/forum/threads/questions-town-portal-to-warp-transition.134834/