A few years back I added an Atlas Jackplate to my 22� Center Console. I am going to sell my Yamaha 150 with a 25� shaft. Can I go down in the length of the shaft from a 25� to a 20� shaft length?

Not if your boat was built for a 25" shaft.

A 20" motor will work on a transom made for a 25" with a jack plate but it won't be optimal. Depending on how deep a v it is and hull design it may work ok in calm water but it wont be low enough for the prop to keep a grip in rougher water. Also every time you come off plane expect the motor get swamped.

Thanks for the info guys. It makes sense that in compensating, the engine won't be able to run optimally.

there is always this if you are getting some kinda "killer deal" ona 20"

https://www.marineparts.com/partspages/Accessories/acc8.htm