In 31 when he didn't even make any solid plan to rescue Tom, given the chance for his Yeerk to die without the other controllers getting suspicious, is when it's obvious that he isn't just waiting until the perfect time to save his brother. He's more concerned with just defeating the Yeerks than saving his brother, which is his main goal in the beginning. 47 is when he admits to himself that he's given up on Tom completely. Two books later he loses his parents and has a heroic BSOD.
I think the major shift happens in his perception of why the Animorphs are fighting. In 18--perhaps earlier?--we find out the Andalites aren't the big saviors that the kids think they are; Earth isn't a major priority. So instead of just holding the Yeerks off, Jake and the rest of the Anis realize they're basically on their own, causing a strategy shift. I think this is when Jake really has to forfeit his hopes of rescuing Tom, though we don't see the repercussions of this until later (and I think Jake doesn't really see it either). Instead of trying to just delay the Yeerks, they're now out to defeat them at all costs. Tom is obviously one of those costs, as is the original reason for fighting for each of the Animorphs. At first they see themselves as partially involved; when the Andalites screw them, they really become the resistance.
I think even though they find this out in book 18, they still retain a childish naivety about the Andalites coming to save them. (I could be wrong, I need to reread!) The actual knowledge that earth is not a priority and the strategy shift that goes along with it takes much longer.
What I like to think is that as the animorphs grow up, there understanding of war becomes more complex. I believe that it is in book 30 when, for the first time Marco is understood to be "ruthless." I reckon he is about 14 at this time and this is simply growing up.
So yeah, I think that it is the knowledge + a loss of innocence due to age and experience that leads to the strategy shift.
