I can change it for my new threads:
But there’s a time limit for post edits: Free to edit post at any time - Feature - Discourse Meta
It seems to be around a month here, perhaps it could be increase even indefinitely.
What’s interesting, I can edit this post from Feb 10 (shown as 30 days ago): Corrugated tube (bellow) - problem with analytical verification - #6 by Calc_em
But not this one from the same day (shown as just Feb 10): Corrugated tube (bellow) - problem with analytical verification - #3 by Calc_em