Next time you want to move something a bit heavy, in fact find something and try this.
Get two lengths of 1/2" round bar and lever one side of what you want to move up and slide the bar under pointing in the direction you want to go.
Do the same the other side.
So basically it's now sitting on two rails, pry bar behind it and it will slide very easily on these 'rails' just before it gets to the end drop two more bars at the side of the existing ones and make sure they don't skid out as you travel over them.
Now if you are using say 6' long bars, that's 12 foot you have moved in a matter of a couple of minutes.
No faffing about swapping rollers over from end to end, can't tip over as it's only 1/2" off the deck. In fact with a drop of oil on the bars chances are you can actually just push the machine once it's got started.
I was show this trick many years ago by on old guy nearly 70 who insisted I not help him and he got an Elliot vertical mill out his shop into the yard on his own just using 4 lengths of 3/4" angle dropped down so the Vee was uppermost and the floor in his shop was a brick floor and not that level.
If you insist on using skates then bolt the Machine to each skate because at that hight if you loose one it's going to go over.

These are my skates, rated at 5 tonnes each skate but I have has a 28 tonne machine on them before. I would never use these on something like a Bridgeport, far too narrow a base for the hight involved. I'd use rails as described above
Edited By John Stevenson on 28/03/2014 23:56:23