Transmission Flush on '03 Impala?

Hello!  First post here - my car is an '03 Impala 3.4L and lately it's been
having a rough time going from 0 to 40mph, it seems to have some trouble
shifting to keep going faster.  The car has 67,000 miles.  No leakage, my
transmission fluid was a bit low so a quart was added but it wasn't super dark
when checked.  No engine lights on.  A shop recommended a flush and
replace the gaskets, I'm not sure if that is a good or bad idea, so much
debating on the net and whatnot.  I believe the manual states a change at
55,000 so I am past that point.  Advice or recommendations?  Thanks in
advance.