Upgrading a treadmill controller / console
Background:  "Long ago" [it feels] I designed the "downstairs" electronics for several different items of exercise equipment which interfaced to consoles and to various sensors. 
I've done some troubleshooting on treadmills.

What you want to do is potentially possible but the factors identified by @knowhow are relevant and some may make life harder than others. It is reasonably likely but not certain that a manufacturer will use similar motor types between models, but also entirely feasible that they may change suddenly. 

Many treadmills use DC motors rated in the "few horsepower" range. There is a reasonable chance that a new controller will drive the motor if both use a DC motor. But, as knowhow notes, if a sensor type has been changed or if a different sense current is fed to a device that is current dependent or ... then there could be trouble. 

If you know enough about how such things work you could add some features to your existing controller. eg instead of hitting "turn off" you could trip a circuit which feeds progressive "slow down" signals to the user speed control inputs to achieve smooth slow down to stop. This could be done entirely at the user interface level and not need much knowledge of internal workings.

If functionality is Ok but the console is physically 'deprecated' you could build a gee-whizz looking console and feed signals to the existing board.

As passerby notes - the control of a motor may be easy enough and being able to "talk to" the controller board may be all that is needed. His two references are worth looking at. They are:
Treadmill motor to drill drive conversion and
similar on you tube