My 240s are both later model, plastic headlight types with the common problem of the seats leaning back further on one side than the other. The wifemobile is particularly bad and gives me a sore back whenever I have to drive it for any length of time. I'd like to replace the seat frames with the earlier type seat frame that is more sturdy. What years should I look for? When did they switch over to the more flimsy design?

1986 and up look for wide headrests.
1985 and before have tall headrests and sturdier seat backs.

Not exactly what you asked for, but have a look at my post re. seat rebuild from July.
http://www.brickboard.com/RWD/volvo-980645.html

Usually it's the foam that disintegrates, giving a seat back that leans to one side - usually angles the driver in towards center. My rebuild cured that for real.

Examining the seat frame can also find loose bolts/nuts holding the back to the seat base. Can also find (sometimes) tilt mechanism not matched on left and right sides. A bit of a pain to square that up but it can be done too.

All this may be useful if older seats in decent condition prove difficult to find.
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Sven: '89 245, IPD sways, electric rad. fan conversion, 28+ mpg - auto tranny. 850 mi/week commute.

I found some really good seats here for $75. It's in Vermont. Don't know if that will help...

http://www.greensborogarage.com/intro_flash.esl