The change in validity does not in any way alter the protocol itself. As mentioned in the linked blog post: if you've already got automated cert renewal, it'll almost certainly require zero change.

After all, the logical approach is to schedule a daily cron job for your renewal process, which only contacts the server when the current cert has less than X days of validity remaining. Scheduling a one-off cron job every, say, 60 days would be extremely error-prone!

With regards to future reductions: the point is to force companies to automate renewal, as they have proven to be unable to renew in time during incidents due to dozens of layers of bureaucracy. 45 days means one renewal a month, which is probably sufficient for that. Anything below 7 days isn't likely to happen, because at that point the industry considers certification revocation unnecessary.

Internal PKI is not relevant here. Anyone with weird internal cert use shouldn't be using public certs anyways, so moving those people to certs backed by self-signed internal CAs is a win. They are free to use whatever validity they want with those.