Is the map going to be updated?
I am not the mapauthor; however since most of us get our road/street data from Census files and they are currently doing yearly updates around Nov/Dec, I would expect not till early next year. I believe WA was done before the Dec, 2008 files were available, when 1/3 of the counties had not undergone realignment.
Do I have to uninstall to go with Above the Timber product?
No. Unless you also select quads/tiles from WA topo, MapSource will create a new gmapsupp.img file which will replace the one currently on your GPSr. Garmin Oregon GPSr can work a little different.
What is the best product out there for good trail and forest service road detail. Also BLM and National Forest locations?
There is no detailed trail data available at the national level. The NWTrail mapset is likely the best you will find. Depends on which State and what source material the mapauthor used.