Strava's RE is a version of TRIMP

It's basically a calculation of time spent in different HR zones, so changing your zones will change the value. I don't think there's a way to change past activities, so you'll just have to wait until the RE graph adjusts to your new zones.

It's exactly as you think, the heart rate zones and how long you are in them in each activity determine the relative effort. If you correct the zones to higher values that means you're activities before were overrated and thus it seems as if you train not as hard now. And you're other assumption is right too, this levels itself out in a short time.

Thank you for the responses confirming what I suspected. I'll just have to keep working at it. It would be nice if there was a way to get Strava to re-evaluate past activities (in a date range for example) against revised HR zones. The only way at the moment seems to be delete the activities and then re-upload them, assuming that you have the original source. But this means you lose any kudos or comments etc.

You can change the sport type and revert that, that reevaluates the RE against the current zones.