So I assume it is the variable pressure shift solenoid that is malfunctioning for you since your 1-2 shift is harder. This solenoid varies the pressure with a PCM signal from the TCM similar to what a throttle pressure cable does on the A540E. Your trans has 4 solenoids, correct? 2 shift solenoids, a lock up solenoid for the torque converter, and this variable throttle pressure solenoid.
You can get that solenoid from here:
http://www.bulkpart.com/Merchant2/me...e=A540solenoid
The one you want is not actually listed on the site I think, but call them and ask them.
Yes, there is only a single bolt that is holding in the solenoid. You just wiggle it out. The new solenoid should come with a new o-ring installed. Just lubricate it with trans fluid and wiggle it back in. There is usually a filter screen on the tip of the solenoids on the OEM parts. For your old one, it may be that there is some junk blocking it after your fluid change. You may want to pull out the old one and just check that. The new solenoid may not have that filter screen. If you are careful, you can pop it off the old one with a small precision screwdriver and reuse it on the new one after cleaning it of course. Otherwise, don't worry too much about putting one back in, especially at 160,000 mi.
cheers.
davemc