Oil-based paint for concrete patio

Grrrr...
For literally DECADES I have been repainting my front patio annually with a dark red oil-based paint from Benjamin Moore. I heard they were phasing it out, so I bought extra cans, but they're now used up.
The local BM store said I have to treat the existing coverage with an oil-based primer before repainting with the new water-based (tfui!) stuff, which is apparently all that's allowed now in California.
This seems counter-intuitive. Why would I have to prime with an oil-based product to prepare for a water-based cover?
Your wisdom appreciated.
(Also, where can I order out of state for the Good Stuff without paying a fortune in S&H?)
TIA
Aspasia
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I've heard rumors that commercial painters can still buy oil based materials. Might do a bit more asking, or maybe someone here knows.
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If you're doing it annually anyway, what does it matter what kind of cheap shit you paste on there?
s
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