Advice Needed, Converting a Boxford VMC 190

[m_c]

If you get enough speed at 16 microsteps, then there's not really any need to change it. One thing to consider, is microsteps should never be relied on for accuracy, as the stepper position can only be guaranteed to the nearest step. While microstepping, both coils are proportionally energised, with the rotor essentially held between two magnetic fields, so can spring/float (there probably is a better word, but I'm tired!) depending on load.

Sensers will most likely be NPN with an open collector. Are there any numbers on them?
If there is, try googling it to see if there are any datasheets for them, which will confirm what they are.