Yeah, the short answer is lots.
The long answer is it depends. There is an element of randomness in where mutations occur, and I imagine it will take several accumulated mutations in the right spots to make the yeast incapable of fermenting at cool temperatures any more. That's my best guess anyway, the S. cerevisiae
(ale) genome is well studied, the S. pastorianus
(lager) genome less so. Any cell that picked up the mutations to prevent it from fermenting cold will only pass those on to its daughter cells, so over a few generations you'll end up with a handful of cells with those mutations, out of billions of cells that are just fine.
Although there are genes in S. cerevisiae
that help it ferment at lower temperatures, I'm guessing that the ability to ferment at lower temps is something that came from the S. bayanus
(wine) side of the family (lager yeasts are the result of a hybridization between ale and wine yeasts) but it's not clear to me how many genes are relevant. Or it's possible there is some novel mutation or gene duplication event that allows for the lower fermentation event, or even some kind of synergistic effect from the combined gene products of the two strains. It's possible this stuff is known, but I'm not up on the literature. If I had research money to study it, I totally would. Anyone got some spare cash lying around?
Anyway, all lager yeasts should grow well at warm temps, you're not in danger of selecting for warm fermenting but not cold fermenting cells unless they pick up some mutation that lets them ferment warm better than the lager yeast does. It's just that you're no longer selecting for only
cold fermenting cells, so you could hypothetically end up with some small percentage that will not ferment cold anymore. Those will go dormant as soon as you pitch into your lager wort anyway, so it's no problem.
I'm going to guess that Wyeast and White Labs both grow their lager strains at warm temps for cell mass. Growing at lower temps would take a longer time to get the same cell count and they'd probably have to charge more for lager strains due to tying up the incubators for longer periods of time.