To improve circulation and maintain a more uniform temp in the house I'd like to swap out the single speed fan and put in a 2 or even 3 speed fan. Would this be a big project to get the fan to work properly? Would the current control board work?
The unit ia a tempstar if that makes a difference

Actually you could put a three speed motor in and then just use a separate 3 pole switch mounted remotely to control the speed a you want them when you want them. Now if you want it to be automatic like slow in heat mode and fast in A/C mode it will become more complex unless you know exactly what you have now and if the circuit board has the switching capability.

I suggest contacting someone with experience to perform this for you.