Strangely enough I have just been trying the very same thing with a 6d Mills Hightop & a 1p Jennings Governor, to 20p. I have given up on both after too many hours.
I put it down to experience & all part of the learning slotties have to go through.
The Mills needed the coin entry reaming out, the escalator guide rivets drilling out & the guides grinding to take the larger coin, the coin tube unsoldering & replacing with a wider tube, after machining the coin top hopper thingy. The coin slides took the 20p coin, however jammed due to the coin not being round.
The next stage would have been to recalibrate the award card as the 6d is thinner than the 20p.
The Jennings is reputably easier to convert
as the tolerances are less keen, however the ally coin entry will need reaming & they don't take kindly to mechanical trauma. The casting is very weak & cracks easily, even overtightening can cause it to fail..
I think the key is, once a machine has been used for a particular coin, it won't take kindly to being altered to take another. Changing from a large to a smaller coin is the easiest option, in my opinion.
You can't buy the bits at B&Q & all manner of unexpected problems emerge as you go along & at the end of the day, the machine will appear to have been 'got at' if the coinage does not appear right, but good luck, it can be done, as there are several people who will undertake the conversion for lots of folding money.