The 808 has a Symbiotik diaphragm. In a nutshell handles more power, less highs.
The 902 should have the same diaphragm as an 802- better highs.

The 802 and 808 are the same magnets. But likely buying working 808's and switching out to 802 will be pricey.

If you get a good deal go for it.
Be aware many used Altecs have knockoff, as in not real Altec diaphragms. I would not pay a lot unless you know the seller well.

Yes 800 series can go to 500. But again, opinion, I think one inch drivers sound better at 800. 416's have no problem doing 800.