Question
 

U have a 265 gallon bare bottom freshwater tank just sold the fish and am thinking about turning it salt. Can I use the freshwater and just add salt or empty half the add new water with enough to turn it salt ?
Also no chemicals were ever used in this tank so I'm not afraid of that I'm just wondering

I don't think you would have an issue. The water column only has about 2% of bacteria in it. You can most likely use that water for your cycle and with water changes through your cycle you should be in good shape.

So, I agree with Wayne on the water. You should be able to use it, BUT...

And this is a really big BUT - Saltwater tanks are setup almost completely differently than freshwater tanks. About the only thing that you might be able to reuse is the heater and the lights.

I would really recommend draining the tank, doing a whole lot of reading, upgrading your hardware for salt and setting it all back up again.

why start with any potential phosphate or nitrate?

Fare enough this is a center overflow with a 90 gallon sump was salt water then fresh for the last 2 months so I'll empty it and clean it first

Agree
I wouldn't take chances.

If you are cleaning water, vinegar and some elbow grease is perfect.