If you're doing nothing more than the basic service interval as specified in the Volvo Service Manual, -then draining and refilling the pan every 20,000 to 30,000 miles is acceptable (with a pan and filter cleaning every 100,000 miles or so). The transmission pan only holds about 3 to 4 quarts of ATF fluid. The whole transmission holds about 10 to 12 quarts total (including the torque converter and tranny oil cooler, both of which don't drain when you stop the car), and so you're really only changing about 1/3 of the fluid by draining/refilling the pan.

On my personal vehicles, I prefer to start with a pan cleaning & filter inspection, followed by a flush with 8 to 10 quarts of Synthetic transmission fluid (Mobil 1, Amsoil, etc), and then repeat every 100,000 miles. The Synthetic fluid doesn't break down like conventional dino-oil based ATF, and it also helps to reduce the internal transmission temperatures. (reducing the transmission operating temperatures by 20 degrees F will double the life of the clutch-packs)

A few observations I've noticed over the years:

1. The transmission filters in the AW-70 and AW-71 transmissions usually don't clog. They're a fine mesh metal wire screen and a simple visual inspection will tell you it's status. These days, I typically don't remove them unless I see particles trapped in them (easily inspected with a flashlight). -not to mention that most of the "tranny filter kits" only have one of the filter gaskets included, and two are required. (there is a spacer block used on 1984 and later Volvos to put the filter lower in the pan, and thus requires an extra filter gasket)
2. The transmission pan gathers and holds all the worn material from the clutch packs. -and cleaning out the pan is far more important than replacing the filter. There is a small 1" square magnet that is intended to hold and collect much of this material, but is undersized for the job. I typically throw in another magnet or two along side the original one before bolting the pan back on. (the large donout magnet from a General Motors T-125 works excellent, and is abundant in the salvage yards)
3. The best transmission pan gasket I've found for the AW-70/AW-71 is available through NAPA for about $4, which is a whole lot cheaper than the $20 filter kit that they offer. It's a rubber gasket, excellent quality, holds well in place without tearing, and can be removed and reused if you have to open the transmission back up soon after your pan cleaning. (obviously merits replacing at the next 100,000 mile pan cleaning due to heat fatigue) You can buy the pan gasket by itself from your local Napa, but you'll have to wait about 2 days for it to arrive since it's only stocked at a few of the regional distribution centers. (think of the savings as paying for 3 quarts of synthetic fluid) I'll post the part number later if you're having trouble finding it.
4. When reassembling the transmission pan and pan gasket, the bolts don't need to be torqued more than 10 to 15 ft-lbs (the book spec is something like 7 ft-lbs). Since a common source of transmission fluid leaks is loose pan bolts, you can keep them snug by applying a small dab of RTV to the bolt threads when you install the pan. Do not use LockTite here, and do not use anything on the filter bolts if you changed your filter (since it can cause problems with the valve body). I usually use Permatex Blue RTV on the transmission pan bolts on my own vehicle, and they hold well until I need to remove the pan again. LockTite is not advised since it will provide too much grip on the threads, and you risk breaking bolts during the next removal.
5. The benefits of using a Synthetic transmission fluid outweigh the higher cost. At $5 to $6 a quart for Mobil 1, the price can be intimidating. However, the cooler temperatures (longer tranny life), smoother shifting, improved cold weather operation, and less frequent changing intervals make it worth the extra cost.
6. A tranny fluid change is also the best time to change the transmission tailshaft bushing if it's starting to wear down. You can perform a quick check by doing the following:
-Properly chock/block the front wheels so the car can't move.
-Place the transmission selector in Neutral (it must be in Neutral to relieve the torque load on the driveshaft).
-Crawl under the car from the driver's side (USA and Left-drive vehicles) and find the back of the transmission.
-Grasp the Universal Joint at the beginning of the driveshaft (just behind the tranny tail flange).
-Shake the universal joint side to side and up and down. If there is play here (movement of the transmission output flange), your tailshaft bushing needs to be replaced. A kit can be ordered from FCP Groton for about $12 using the part number 235878KIT.

God bless,
Fitz Fitzgerald.
--
'87 Blue 240 Wagon, 262k miles.
'88 Black 780, PRV-6, 149k miles.