"RB" wrote in message: "Got a trailer with an 18' ProLine, 135hp o/b, and 20
gal fuel tank sitting on it. It has a 2" hitch. I'd like to change to 1
7/8" hitch. Any reason not to?"

You have a few options.
Change all your trailers to the same size. I would go with 2 inch over 1
7/8.

If you have a class 2 or 3 receiver/hitch you can buy another hitch and ball
that will slide into receiver with the correct size for what ever you pull.

If you have a class 2 or 3 receiver/hitch you can buy another hitch with
multiple balls mounted on it. You then just rotate the hitch in the
receiver for the proper ball size needed.
For example see http://www.trailerhitches.com/ball-mounts/

If you mount on teh bumper then you can buy a ball with shaft that allows
you to change ball sizes.
For example see http://www.trailerhitches.com/hitch-balls/

Sarge