I can now say that the effects indeed only apply to newly placed buildings.
But the main reason I'm commenting again is because I've hit a point where Rampant has gotten a bit insane. It has started spawning a variant of bug that shoots electrical lasers, is insanely resistant to lasers, and somehow throws down "energy absorbing crystal pylons" after destroying buildings, which are also super resistant to lasers. After several thousands of bullets I managed to wipe out their base, but wow am I getting nervous about what other crazy stuff will spawn.
Also, space exploration + Krastorio 2 is fucking insane. The technology get's a bit weirdly paced around the space part, but the main problem is that I have to figure out a way to automate the delivery of certain items to space for research purposes. I'm also seeing concerning accounts of running out of all resources on the planet...
SE/K2 is intense. The tech getting weirdly paced is because the compatibility for the two isn't fully complete - there will be some oddities but it should be fully playable.
Your options for automated delivery are going to be cargo rockets at the moment, maybe delivery cannons - can't remember when those unlock. Head on up when you've got the chance and bust through what you can manually, if you haven't already. Automation of transporting items is going to come from the circuit network, using transmitters and receivers from AAI. Have inserters fill the rocket, have it pointed at space, and have it launch at full cargo. If you're not sure how to do that, the
discord is a good place to look around (beware of spoilers, they're usually spoiler'd but things happen.)
As for running out of resources...
do some core mining. Diminishing returns are irrelevant when you don't have the resources elsewhere. Your only concern is going to be dealing with the resources (they come out mixed) and power draw. Core mines never run out of resources.