I don't have RPG maker on this computer, so I can only try to explain using my words :)
What you'll want to do it use events. Tileset graphics are listed down at the bottom when you set event graphics.
The portal looks like it has six tiles, so you'll need six events, one for each tile. I am assuming that your "off"
portal is one the map as tiles (which is to say: don't delete it)
Designate a Control Switch as your Portal switch.
For all six events:
On the first event page have the page condtion be when Control Switch "Portal" is OFF
Set the event as "Same as characters" and "Direction fix", walking and stepping animations set to OFF.
Set the graphics on the first event page to NONE. When the portal is off, it just shows the original tiles.
On the second event page have the condtion be when Control Switch "Portal" is ON.
Set the events as "Same as characters" and "Direction fix", walking and stepping animations set to OFF.
Set the graphics on the second event page to those of the "on" tiles.
Now, when you want to switch back and forth, change the Portal switch on and off as needed. All six events will change at the same time. When the switch is off, the events will be invisible, when it's on, they will show the on graphics instead.
It can sometimes be easier to make just ONE event to test such behaviors, and then once you have a template, make the other five.