There's no need to do the service again until the next maintenance interval. You've already done what's expected. Doing it again will take you from about 50% new fluid to about 75% new fluid. If you're doing the work yourself, it's cheap and really can't hurt anything, either.
Your filter bolts onto the valve body. It cannot be dislodged unless the bolts are loose.
Anyone that does flushes can do it with SPIII. They may not wish to deal with the dealer to get the fluid, but in that case, you could purchase the fluid and take it in yourself. But don't think you need to do this, either. Once you've replaced the filter and put new fluid in, you've met the maintenance requirement.