But only if they all switch.
That doesn't actually make any sense.
If Texas switches to proportional voting, then says it's now 55% Republican and 45% Democrat. Democrats can then look at it and realize if they allocate a
little extra money to Texas, they might get that up to 47%, which would then add to their national vote, so it would matter. However, then Republicans would see that as a threat so they'd spend more in Texas too, and an arms-race would entail with both parties ending up prioritizing Texan voters of all stripes more. What other states are doing at this point in time would be completely irrelevant.
Whether or not any other state switched to proportional wouldn't matter. In fact, if
only Texas switched to proportional, then it would matter
more not
less. Since, say both Texas and California switch to proportional, then you'd have to decide how much money to spend in California and/or Texas, so you're priorities would be split, so the money would be split. However, if
only Texas switched then it wouldn't matter if Republicans spent $1 or $1 million dollars in California, so they wouldn't spend anything there, but Texas would be competitive. So not only is it not correct that this only kicks in "if they all switch" it is in fact more beneficial to be the first one to do it, if nobody else does it.
If some don't then those that don't get more of the funding
I really think you just don't get it. Democrats don't spend
anything in Texas now and Republicans don't spend
anything in California, since no matter how much money they spend they're not going to flip those states. If other states switch to proportional then the winner-take-all states that are solidly blue or red don't get money "reallocated" because the situation is still the same - no amount of advertising spending in California is going to get a majority voting for Trump, and no amount of advertising spending in Texas is going to get them voting for Biden.
However, if a state is proportional then a proportionate amount of money spent will pick up
some votes, reliably. Winner take all states are all or nothing, so unless they're right on the knife's edge, neither candidate bothers with them right now. But say if Texas goes proportional then spending a few million will pick up a percentage or two, it would be a better bet in terms of risk/reward than doing a full on media blitz costing billions of dollars trying to flip a hardcore red or blue state.