Tested with a quick test script: https://gist.github.com/bwplotka/cb4eb538850cd9bf35de52c708037a6d

I think there should be an issue filed which says what goes wrong if we don't merge this.

It is hinted at in #14749 ("Is downgrading from v3.x to a pre-v2.55.0 version completely unsupported").

Since you tried it, @bwplotka, can you file the issue?

This is similar to #14939 but expanded to our current LTS release 2.53

The diffs seem identical to me; what is "expanded" ?

I think there should be an issue filed which says what goes wrong if we don't merge this.

A quick 2-minute test says it works fine. Going to guess that we need a slow test, i.e. with some blocks created after a few hours.

Update: I added --storage.tsdb.min-block-duration=65s to make block creation happen faster (idea from #16550), and it still starts up without error after downgrading from v3.0.1 to v2.53.4.

What will fail: If we actually execute on #14749 (change index version) you will see failures on this LTS and generally before 2.55 (we ported all above)

Built quay.io/bwplotka/prometheus:v3-index that simulates Prometheus with this issue completed.

Ah, thanks. That wasn't clear to me.

Can you please answer the question on #14749 ("Is downgrading from v3.x to a pre-v2.55.0 version completely unsupported").
(I think the answer is it is supported for all x from 0 to 5 at least).

Good question, i answered on the #14749 (comment)

...so, are we ok to merge this @bboreham ? (: