I don't suffer quite as badly from this particular anxiety, though I do have a whole host of worries regarding my comic that keep me awake at night sometimes. Posting ANY kind of art online is an act of vulnerability; you're putting something that matters a lot to you on display for strangers, and there is NO way of predicting how they will react.
Ultimately, though, it is better to just do the thing. I've found that as scared and nauseous I get at the thought of doing the thing, doing the thing has always proved to be not as bad as I thought it would be.
I think that if it worries you, you might want to mention it to your readers, and be honest that the tone of the series will be getting darker. You won't have to spoil anything - just say that hey, things might get dark from here on out.
It's probably fine if you don't, too; I imagine your comic won't go from sweetness and light to utter darkness in the span of two pages, so you will have time to hint at the change in tone, build up to it in your story, and it will probably just feel natural to your readers when it happens.