I have not seen one of these plugs to know if it can be converted. The main concern is not the widening of the primer channel, but the depth. It would depend how much metal is between the seat of the 209 and the face of the plug inside (base of combustion chamber).

Only you can make the decision if you are willing to try it and take the chance that it is still safe! You could check the Precision Rifle website to see if they already do it, that would be a good indicator.

If you already have a lathe, just get some cases to get the diameter right and use a caliper to measure the length of your plug with the 209 primer seated in it. Then drill out the plug until the length with a 25ACP case inserted is exactly the same.

I did one of mine myself, I did not have a case on me and was trying to do it an inferior way and the depth is a little short and needs to be redone...