two sites one domain problems

Hello all!

My problem is:
There is old site at the address:
http://isop.pl
It was mutlisite, but I managed to switch this feature off.

We are building new site in http://isop.pl/new – a new WordPress and etc.

Now, I was able to make subdomain: old.isop.pl and point it to / where my old site is located.
However once I try to point isop.pl into /new – it doesn’t work.

I will appreciate if anyone enlight me… I have no idea…

Most I can do is to make http://isop.pl/new to work. I though I may change Site Address (URL) to http://isop.pl at least instead of http://isop.pl/new but as soon as it touch here – new site falls apart.
Even if I change here back to http://isop.pl/new – it does not want to start back again…
I have no idea how to handle that issue…

Tnank you…