This is just an opinion, but:

Flush it. If it craps out your trans, your trans was screwed anyway (trust me on this).

I would do it yourself if you have the knowledge/expertise, and I'm assuming that you do. My Volvo flush kit is a retired OMC fuel hose with primer bulb attached still. It works perfectly. I just did my dad's 940 with 250k yesterday, but I do it every 15-20k since waaaay back when.

If you have no ill effects after flushing and driving a few 1000s of miles, I would consider using the opportunity to "upgrade" to synthetic. Mobil 1 makes a good one for this purpose, and it stays FRESH for a very long time.

Having done work for a Volvo shop previously, I did a lot of this. Nobody was unhappy with it, but if your fluid is filthy/black/smells like varnish, you are probably not in good shape to begin with. The upshot of this situation is, used AW70 transmissions are all over the place anyway, so they cost little.

FWIW I just did my Toyota Avalon with a full flush, then another full flush (after 5000 mi) with synthetic, at 75k. Its a happy trans. The AW unit in the Volvo is better than the unit in the Toyota/Lexus cars, so if it is still in decent shape, flush it.

One word of caution: The trans may not shift ANY smoother when you flush it, in fact it may shift MUCH harder. This is not an indication of a problem. The AW transmissions eventually often (1 out of 2 usually) inherit a hard shifting pattern and it is lessened by dirty fluid in my experience.

-CH