What the diff between bravo one and X orXR,can bravo one be upgraded?

Do a search here or go to Mercury's website which details the differences under the specs for each drive.

Bravo 1 and Bravo X not a lot of difference.
Now the XR is a lot more robust.

Here's some more info. to help clarify;
http://www.go-fast.com/mercruiser_bravo_sterndrives.htm
http://www.go-fast.com/bravo_details.htm

I run XRs, but own a pair of XZs as spares that are 2 MPH faster. I learned this after I put the XZs on last year. Both are great drives

Yes the XZ drives have spiral cut gears and the XR drives have straight cut teeth. Other then that they are the same drive and it is easy to convert one to the other by just changing gears. The spiral cut turn with less friction and rob less horsepower which makes them faster, but the XR's are stronger.

Both the XZ and XR have larger prop shafts the the standard bravo also.

I think the X is an upgrade from the standard Bravo but I do not know the difference.

Old Bravo1 had thin floored gears, would break out when power was too much. The X gears have thicker floors in them. The Bravo1X has a steel tower in the case..
Doc, have the uppers done, working on the lowers..
Any takes on that Baja yet??

Dick