internal filter does not need to be replaced. Allison/GM say only to change the internal filter if work is done on the transmission. Since the pan has a drain plug, no reason to drop the pan.
Next on fluid, If you use Dexron XXX then fluid change is at around 50K miles. And GM fills the transmission with Dexron. Now if you want extended drain intervals, (100K+) and better protection have it filled with Allison Transynd. After one fill you will need to change it again at 50K, then go to the extended fill. Transynd is expensive but I found that here NAPA stores have a castrol branded transynd at a reasonablle price.
Allison recomends against a flush, So if you want go buy a Allison spin on filter and fluid and to the change yourself. Just drain, replace what you drained, check level cold to be in the lower hash marks, then go for a drive to get the fluid up to about 180F and check again and top off if needed.
