In industry, if the ACME widget company buys a new widget making machine, the employees wouldn't be bonded for training on the new machine.
Perhaps not the current employees, although this has not stopped one company I worked for.
Any new employee would be bonded within the T&C's of the contract much as the current employees would have been when they started.
I don't see how a bond can be legally enforceable if you have not agreed to it in a contract and it wasn't made clear to you that a bond would exist when you started training.
In the two cases where I have been bonded the requirements of the bond were clearly explained in the contract, which was required to be signed prior to the commencement of ANY training.