ok im with you... you will need to exsperiment with your motor tunings to see what the machine will run at with the new fittings.
keep upping the top speed until your steppers start stalling then back off the speed by maybe 20%
you will also need to do some mathermatics to work out how may steps per unit and update that in motor tuning (you can do this automaticaly if you have a dti but its probably better to get the calculator out and do the maths:)
let us know if you get stuck :)