It's very easy to swap between the II, III, and 475 boards. In fact, where I went to school, we swapped several of our IIs for IIIs just in board. (We also had to buy hard drives because we we bought the stripped-down LC II without them and the III, of course, requires 7.1).
Speaking of which...if you upgrade, be aware the LC and LC II run fine with System 6.0.6+ and 7.0/7.0.1, as well as 7.1 with no enabler...but the III and 475 both require 7.1 with an appropriate enabler. It's 003 for the LCIII (easy to remember, put 3 and III together!) and 065 for the 475 (also easy to remember, rearrange and you have 605, which is the Quadra equivalent).
The LC II actually has the contacts for the LC I speaker assembly. Look up the board. The III lacks this. I think I actually have a II board with the speaker assembly from the I in one of my LCs...
One other word of caution: the RAM is different between the I/II and III/475.