As technology evolves rapidly,  it is inevitable (and recommended) that you will want to bolster your company's network infrastructure  with the newest and best stuff out there.   Getting the best possible network hardware and technology will mean a heartier infrastructure, better security, and speedier collaboration.   Although updating your network hardware and software is best practice, it is very important that the process is done right so that your expensive network doesn't crash and burn when you need it most.

Although it is pretty much expected that you will have some network downtime during the lifetime of your setup (varying on how complex it is), there are several things that you can do to minimize this.   Especially today, when saving capital is of the utmost importance for business, preventing any network downtime will save your company a headache as well as loss of precious revenue.  The following is a list of steps you can rely on to minimize the chance being blindsided by a total network blackout:

1. Plan for compatibility: Just like you want to learn about a date's likes and dislikes to see if there is potential for long term compatibility, you also need to carefully plan the integration of new and updated network hardware into your older structures.   This can be done simply by doing some research on the network hardware products with good ole' Google search.
2. Testing is King : Create a near identical testing environment that mimics the network environment to give it a test run before going live.   It can be very dangerous to go live with new network hardware without 'beta testing' any potential problems and flaws.   Although it is very difficult to create a live testing environment without utilizing real-world stress, it is important to try to do this as diligently as possible.
3. Stagger Your Release: Like any control-variable experiment, it is important that each addition is installed and tested individually and sequentially.  If you add multiple variables at once, and something goes awry, it will be impossible to determine what caused the problem.  If each step is implemented and tested against a control, it can easily be determined at each interval what is working and what isn't.
4. Be Ready for the Worst Case Scenario:  Expect that on the implementation of your new network hardware there will be some downtime.  Be prepared for this by creating a quick 'reboot' protocol, a blueprint that you can follow to get the network live as rapidly as possible.  Also schedule your implementations for off-hours when it won't have a major impact on your business users.
5. Finish twice: When you think everything is up in order and working fine, go back to the drawing board and re-test everything.   Usually there are one or two components that you haven't predicted and will cause a problem down the line.  Your due diligence during installation can prevent a hassle and loss of capital in the future.

Hope this helps.  As you can see the important part is preparation, and as in anything, the more the better.  Another important part of the equation for success is to to make sure your network hardware products are the best available on the market.  This is easier said than done, especially when small businesses are trying to save every penny available due to the economic downturn.   I recommend utilizing a reputable used network hardware reseller, where you can purchase equipment at a fraction of the cost.  Make sure they offer a good warranty and be sure to get their customer service on the line to answer any of your questions before going forward with a purchase.