After I upgraded my USR external to v.92, a lot more of my connections slowed and sometimes even became v.34 instead of v.90 or v.92. I wouldn't upgrade unless my ISP supported v.92 and I could downgrade (I can't).
I found that v.90 USR externals that can be upgraded to v.92 are $10-20 cheaper than the same models that come from the factory with v.92.