I hope this is not a stupid question but can you change the lower unit gearing by swapping them out with different ratios or do you have to get a new lower unit with gearing already in place?

You can change the gears inside the lower.

XR uppers, the ratio can only be changed by changing
the lower gearset.

Reg bravo/X, are changed by changing the
upper gearset ( although, there are a few different
tooth count lower gearsets for bravo 2/3's that
could make up some different combinations)

You can put the XR lower gears in a B1 and change the ratio if you choose.

Thanks for the replies guys!

We do have the X drive, I forget to mention that in the first post. As well as the 496HO stock.

Now my second question... what would going from a 1.5:1 to 1.65:1 do to the boat? Mainly any effect on the top end/speed. Are there any other advantages to changing the gearing?

Thank again for the help.