First of all, 802.11ac is defined only for 5GHz band, 2.4GHz band will continue to use 802.11n. Higher frequences fade more quickly, so you will need more 5GHz access points, because they need to be closer to each other. The 5GHz band was already in use with 802.11n, so this has usually already been taken care of.
The new speeds in AC use wider channels (80MHz and 160MHz) and more precise modulation (256-QAM). High precision modulation requires very good reception, in practice it requires clear line-of-sight to the access point. You won't get AC-speed if the access point is on the other side of a wall. This is another reason you usually will want more access points when upgrading to AC. You also gain more capacity so the network can support the increasing number of users and their requirements. When you add more access points you need to turn down the transmit power so the access points won't interfere with each other, otherwise adding access points will degrade performance instead.
The third challenge is more technical. AC access point require more electric power. If they have their own power supllies this won't be a problem. If the power is fed through the Ethernet cable from the switch, the switch may need to be upgraded. Only the latest 802.3at (aka PoE+) can feed the power hungry AC access points. The older standard was 802.3af (aka PoE without a plus).
You may need to consider the data bandwidths as well. 802.11n of the today already exceeded the capacity of 100Mbps Ethernet. 802.11ac requires at least gigabit Ethernet connection – the more powerful access points have two Ethernet ports, because at least in theory you may exceed the capacity of one. In any case dual cabling will add redundacy, but will require more switch ports. If you have connected multiple access points to the same switch, you may need to upgrade the uplink to network core as well to avoid bottlenecks.