You can only port it so long as the old service is active. I tend to port before I cancel in case the company gets any ideas or makes it difficult to close my account -- that way if I have to fight them over the bill I still have the number with the new company.
That is to say, if you cancel your old service before completing the port, you are out of luck.
There's also no reason your new service can't port in the number, but you didn't ask about that. Just wanted to mention.
Don't take the previous owner's word for it for sure, but I don't fault you for looking at options if the internet available doesn't work out either.
Can you test the connection ahead of time at the new location?