Lower voltage will make lesser demands on the existing ESC so you are safe there.

You'll get less power if you don't change the prop.

If you DO change the prop, you may get close to the same amount of power, but you'll have to choose carefully.

The decrease in power with your existing prop will be rather dramatic however, so you will not be satisfied.

You COULD run your 7S pack with a 3S pack ( of the same capacity ) in series to make up the difference and end up with an effective 10S.

Likewise you COULD run your 6S pack with a 4S pack ( of the same capacity ) in series to make up the difference and end up with an effective 10S.

In your shoes I would do either of the latter.

However you'll need to charge and balance the batteries separately.

Remember that the add-on packs must be of the same capacity as the ones you have now.