As Mike said fish will spread out and grow (numbers) to fill the area. But you have some choices to make which could improve the situation. In addition your population will change during any drawdown .

First are you sure you will have enough water ? Lake drawdowns can have a positive or negative effect on fish dynamics depending on the situation. As they are crowded into less water the predators have a feast. But to many fish in not enough water can cause water quality problems and fish kills and can deplete your forage base. Drought can add to the problem by reducing already low water to none. Will you have enough water left to sustain your fish and to refill the enlarged pond in a reasonable time?

It sounds like you are describing a LMB crowded pond (many 10in LMB). That is a common problem which can be improved during the enlargement process by adding the right type and number of fish. A pond with LMB ,HBG and CC is not self sustaining as it does not have a sufficient forage base. HBG only produce a few offspring because of their skewed sex ratios where BG reproduce many times more and can be self sustaining. The result of your combination of fish will most times result in a LMB crowded pond over time. There are also other forage fish options to think about.

There are some threads here on pond enlargement, drawdowns and the effects on fish populations and management options. First you should think about and tell us what you want out of the pond (your goals). That will help you to focus on what you want and will tell us what suggestions and explanations to make.

The more info you can provide the better and surveys (there are various ways and methods)are how you get that info. Most times it is not necessary to just start over as the existing fish can be incorporated into the new plan , unless you have unwanted types (that can't be managed around) or bad genetics in the existing population.