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.