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I know that we need to go through the bed-in process when installing a new disk, but what about when changing only the pads?

Technically, the surface of the disk has already been prepared with the original bed-in process, but I was wondering if there would be reasons to repeat it. Maybe that would help rejuvenating the disk surface or getting the brake pads surface at the right angle.

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Yes. The bedding in process also prepares the brake pad's surface at the same time.

No need to make it complex, just do a set of hard but controlled stops down from a decent speed to stopped. Avoid mandatory stopsigns or red lights, instead do it on a quiet straight road with no one behind you.

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    If, like me, you avoid driving (especially to ride) but live in a busy area, you can still do it: build up some speed then for the junctions slow down sharply on one brake, easing off as you come to a stop. It's not quite as good but works in practice. Refinement: slow down most of the way on the front brake, then do the last little bit of stopping with the back; once the front is done, bed in the back. Further refinement: bed in the front with the old back fitted, and used for non-ideal braking. Then change the back pads and bed them in, using the front as needed.
    – Chris H
    Feb 1 at 11:35
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    @ChrisH My preferred method for bedding in brakes is to simply apply the brakes as hard as possible while still letting the bike move, and ride around for a bit. You can feel the braking power increasing as you go haha.
    – MaplePanda
    Feb 1 at 19:56
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    @MaplePanda That's a good approach, or sometimes just a good start. I can do that down my gently-sloping road but not finish the job going up. In fact I can bed without leaving my road by taking your approach up, going down fast with a couple of near stops cruise up without braking, down once more.
    – Chris H
    Feb 2 at 9:02
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    you can give yourself a head start by pouring some water on the fresh pads and rubbing them together for a bit. You'll feel them start to grip after <1 minute. Rinse off the grime you've created before installing.
    – Paul H
    Feb 2 at 17:08
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    I just watched a video from Shimano about the bed-in process, and while doing it, they recommend "not coming to a full stop since it can leave inconsistent deposits of pad material on the rotor's brake surface leading to noise or pulsing during use". From your experience @Criggie, have you noticed anything like that while bedding-in your brakes?
    – olliebulle
    Feb 4 at 14:47
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The bed-in procedure is meant to create grooves in the disks corresponding to elevations in the brake pad. As well as abrade the brake pad itself. This ensures a large contact area and avoids local overheating.

It's a one-to-one relationship, one rotor to one set of pads. Replacing either of the sides requires establishing the connection all over again.

The surface of the disk has only been prepared for your previous pads, not any others.

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