@JauntyArt You can go in & add hashtags into old posts via the Edit option. The only down side is that anyone who has liked or boosted it gets a notification (obviously in case someone changes a post to something unpleasant)
I did it half way through a weekly series but put a post up that said what I was going to do, so people would know why they reappeared in notifications! I'm sure not everyone read it, but it meant it was sitting in my timeline if anyone was puzzled.