A delayed 1-2, or a harsh 1-2 upshift can by symptoms of a problem with either the spring or accumulator (piston).
You do not have to drop the valve body to access these.
Any REPUTABLE tranny shop that you trust can easily swap out these parts in less than an hour. 30 minutes would be do-able.
You should visit
www.tccoa.com
Go to the Tech Articles, and in the Transmission section, read Transmissions 101, and the other two articles about the tranny.