lab relations and busy professor
As you are someone who tends to need structure and, perhaps I could even say hand-holding, and since your target advisor has a set-up that is more amenable to that style... you might be able to get some assistance from the target advisor in making the change.

You could also ask the director or dean of graduate studies in your department for assistance.

Alternatively, you could take a deep breath and let your current advisor know about the change.  Use I-messages such as "I'm the type of person that needs to be pushed a bit more or else I tend to procrastinate."  Make sure to thank the professor for the opportunity s/he provided you.