An old trick used to be, you took an old, garbage keyboard and gutted it, so it only had the cord and controller, wrap that with electrical tape and toss it into a dark corner behind your computer. Leave that plugged in until you needed to swap keyboards. On some boards, you could use the mouse port for the dummy board (I'll talk more about this is a second), and then use the KB port when needed without unplugging the dummy. This doesn't work on all os or motherboards though.
There is a down side to all of this, and that is the lifespan of the ps/2 port, they were not designed for repetitive connecting and disconnecting, but then, neither was USB A. If you have a dummy and only one port, it means twice the wear.
Whatever you do, do not repeatedly plug into the port furthest from the motherboard pcb, there's more leverage, and it can cause the bracket to wiggle and cause that port and the lower one to no longer work. And of course, this says nothing about what it's doing electronically. Newer ones are probably safer as they have lower amperage (which is why they can't power a Model M), but they still weren't really designed for hot swap.
Also, both ports were not always created equal. Even back when they could power Model M's, the mouse port often had less power. So some keyboards would only work on the keyboard ps/2 port, others could work on either.
If you can use an adapter, that's probably the safest bet.
