So, I've been following this series as its been released on here for a few years now, and there is clearly a good degree of character/relationship growth going on. Haruko and Natsuko have gone from complete strangers, to friends, to best friends and maybe a little more than that. However, this series has been a cute and funny 4-koma series from the start, and it's pretty likely that the author is just going to keep it going in that direction.
As has been stated by others, the moment Haruko and Natsuko enter a relationship, that would quite possibly be the end of the series. The author clearly wants to keep the series going, so the relationship between them has been kept at a kind of half-way point for a while now.
I feel like a lot of the comments lately are either, they won't end up in a relationship because subtext, or they are basically already in a relationship because goggles. This is a slice-of-life and comedy series that explores the idea of being able to swap bodies with somebody, and that's what it's always been. Yes there are hints of romance because of all the kissing and stuff, but the romance has always been secondary to the comedy, and that isn't really likely to change.