How safe is port changing?

I need to setup RDP on another server in my network. Currently, i have all traffic on that port forwarded to a specific server. I want to setup another port to point to 3389 but to a different server. How secure is this?How safe is port changing?
Actually, as long as you make sure you're not interfering with another application, it's more secure, for a couple reasons.

The biggest reason security increases is that it's uncommon to change port numbers. This means that if a hacker is looking to get into your network or web server, they'll likely scan common open ports, such as 20, 21, 25, 80, 443, etc. Especially with high ports, they don't bother scanning each one (unless they're very determined). With thousands of ports on each computer, it's only efficient for an attacker to look at a dozen or two and judge the computers security based on that.