Multi-input multi-output (MIMO)
A: The older Wi-Fi systems 802.11a/b/g transmit a signal from a single antenna. In 802.11n, the higher data rates are achieved primarily by the introduction of new antenna techniques referred to as multi-input multi-output (MIMO). When you change antennas, the coverage pattern and range changes.
Upgrading to 802.11n generally improves the range of the signal by 10 percent to 25 percent. In other words, the signal from an 802.11n Access Point (AP) will travel further. This means that if you simply replace your 802.11b/g radios with 802.11n radios, the APs that are deployed on the same frequency channel will interfere more with each other. This interference could cause a significant degradation in performance.
Radio frequency plan solution
Redoing your RF plan will enable you to avoid these problems. The solution might be as simple as reducing the transmit power of the 802.11n APs, or you might be able to reduce the number of APs in your network.