The ladies have given you great advice!
I agree that one guide prepares a child well for the next guide in line. If Beyond has been a good fit for your pair, then it is likely that Bigger will be too. You may wish to begin Bigger Hearts at half-speed for a couple of weeks to train your kiddos in the new aspects of the guide.
Then, you can bump up to full-speed. Since you have a 7 year old, you could consider doing Bigger just 4 days a week with both kiddos. This would mean that you would go 9 additional weeks into the next school year to finish Bigger, before beginning Preparing. This would allow your young one to grow up a bit, without holding your older child back. Plus, you'd get a free day every week, which is a huge blessing!
I'd love to hear your thoughts when you get a chance!