Can +48v DC Voltage be achieved by reversing the terminal of -48v DC voltage supply?
In most cases a telecom -48V PSU does not connect to chassis neither the positive, neither the negative terminal. However it is good to check this first if the device you will power have ground (earth) connections through other cables.

If earthing is not a problem, there is no other reason not to use this power supply. Just connect its (-48V) terminal to your GND and its 0V terminal to your +48V.