Go ahead and change everything you feel like. It's your comic and you should feel you have full control.
If it's just spelling then you really should.
If you're replacing art you might want to consider notifying your readers in a comment for your next episode so the ones that want can re-read.
If it's story, names or similar then you can select to "notify" your subscribers when you save the changed episode/s. You should also leave a note at the end of your next update or even make a special update notifying that you made changes.
The perks of electronic toons. ^^
Now, for the issue of making it a habit. Maybe that's just someone's style, a dynamic comic that evolves even after episodes been published. However, as a reader I'd be really annoyed if it happened more than once or twice when it comes to story and plot. A good story flows without loads of reboots. The only reason I'd go back and read first episodes again would be to check if that awesome plotwist really was nicely foreshadowed and set up at the beginning. 