What will deleting a full month's transactions do? (edited)

Well, the short answer is that it won't affect your balances unless it's a manual account. I assume June is the first month for which you have transactions. If that's the case, then you can start where you wish.

But why would you want to delete them? If you have errors, and really what errors you have don't matter much right now since you are in the August spending plan. The more data you have going back, the more the reports mean and the more you can compare spending patterns.

So I wouldn't delete anything. Just keep marching forward.

You're welcome. Also, I meant to say that if and when you discover an error down the line, you can just fix it. I sometimes find one when doing reports; usually that isn't categorized right, and I fix it. As long as the amount and the account are correct, how you categorize or split is up to you.